Once More unto the Breach, Dear Friends

Having promised to deliver a (very) quick analysis of the UK general election, I guess I had better deliver.

There are 650 electorates to consider here—more than 4 times the size of an Australian federal election—with three countries and a principality to consider: England, Scotland, Northern Ireland and Wales. Fortunately for the speed-psephologists of the world, but unfortunately for the British, each seat is determined on a first-past-the-post basis. This means we can generally consider the two main contenders in each seat and ignore all of the others. This is what I will be doing for 568 of these seats. I have made 79 exceptions where the third-place party placed within 10% of the second party in the previous election (as these might need a three-way consideration of likely winners); an exception for Belfast South where the two main parties combined held less than 50% of the votes and thus cannot be considered entirely safe; an exception for North Down which is held by an independent (as swings to independents cannot be calculated without seat-specific polling); an exception for East Devon where the second party is an independent (for the same reason); and an exception for Buckingham as this is the speaker's seat (and as Jay Foreman, informative stand-up comic and one half of the Map Men, explains here this seat is uncontested by convention (except UKIP and the Greens sneakily contested it anyhow last election (and are doing so again this time))).

Taking these 568 seats, however, we can consider the two main contenders, the number of votes each held last election and then calculate their percentage of the two-party vote (2P%):

This boils down to 18 different contests. I've listed each below, the number of votes each party won in total in the previous election and the 2P% of the first party alphabetically. I have also taken the latest ICM/Guardian polling (more recent Opinium polling is now available) and worked out the two-party share held by the first party. The difference between the two percentages is the swing in percentage points.

CONTEST			LAST ELECTION (VOTES)			RECENT POLL (%)			SWING
Party 1		Party 2	Party 1	Party 2	Party 1%	Party 1	Party 2	Party 1%	Percentage Points
Conservative	v	Labour	11,299,959	9,344,328	54.74%	45	34	56.96%	Conservative	+2.23
Conservative	v	Lib Dem	11,299,959	2,415,888	82.39%	45	8	84.91%	Conservative	+2.52
Conservative	v	Scottish National	434,097	1,454,436	22.99%	27	40	40.30%	Conservative	+17.31
Conservative	v	UKIP	11,299,959	3,881,129	74.43%	45	5	90.00%	Conservative	+15.57
Conservative	v	Plaid Cymru	407,813	181,704	69.18%	35	8	81.40%	Conservative	+12.22
Green	v	Labour	1,157,613	9,344,328	11.02%	3	34	8.11%	Green	-2.91
Labour	v	Lib Dem	9,344,328	2,415,888	79.46%	34	8	80.95%	Labour	+1.50
Labour	v	Plaid Cymru	552,473	181,704	75.25%	46	8	85.19%	Labour	+9.93
Labour	v	Scottish National	707,147	1,454,436	32.71%	25	40	38.46%	Labour	+5.75
Labour	v	UKIP	9,344,328	3,881,129	70.65%	34	5	87.18%	Labour	+16.53
Lib Dem	v	Plaid Cymru	97,783	181,704	34.99%	5	8	38.46%	Lib Dem	+3.47
Lib Dem	v	Scottish National	219,675	1,454,436	13.12%	6	40	13.04%	Lib Dem	-0.08
Alliance	v	Unionist	61,556	184,260	25.04%	9.8	28.8	25.39%	Alliance	+0.35
Sinn Féin	v	Social Democrat	176,232	99,809	63.84%	27.9	13.7	67.07%	Sinn Féin	+3.22
Sinn Féin	v	Ulster	176,232	102,361	63.26%	27.9	15.8	63.84%	Sinn Féin	+0.59
Sinn Féin	v	Unionist	176,232	184,260	48.89%	27.9	28.8	49.21%	Sinn Féin	+0.32
Social Democrat	v	Unionist	99,809	184,260	35.14%	13.7	28.8	32.24%	Social Democrat	-2.90
Ulster	v	Unionist	102,361	184,260	35.71%	15.8	28.8	35.43%	Ulster	-0.29

Note that contests involving an exclusively Scottish party (in thistle purple), Northern Irish party (in shamrock green) or Welsh party  (in leek... beige?) do not have meaningful UK wide 2015 results or 2017  polling. These are compared with the votes and polling of the other parties only within their respective countries: sources for voting in Scotland, Northern Ireland and Wales. New polling for Scotland was available at the time of posting.

Applying this two-party swing, or modifier, to the 2P% in our first table yields a quick prediction:

Now for the exceptions

To deal with the easiest first, Buckingham is almost certainly going to stay with the speaker because that's how the Speaker's seat is. The independents in North Down and East Devon are both contesting their seats, so we can expect a comparable contest with 2015. In both cases the winning candidate held almost 50% of the votes, with their nearest rival being around half that. I think it is fair to expect no real change in those seats on those numbers. That leaves Belfast South and the 79 seats where the 3rd party is within 10% of the second.

In these 80 seats, we can apply a similar method to that we used for 2 party races, but based on 3-way splits of the vote (the 3P%) and the following calculations:

CONTEST			LAST ELECTION (VOTES)						RECENT POLL (%)						SWING
Party 1	Party 2	Party 3	Party 1	%	Party 2	%	Party 3	%	Party 1	%	Party 2	%	Party 3	%	Percentage Points
Conservative	Green	Labour	11,299,959	51.83%	1,157,613	5.31%	9,344,328	42.86%	45	54.88%	3	3.66%	34	41.46%	Conservative	+3.05	Green	-1.65	Labour	-1.40
Conservative	Labour	Lib Dem	11,299,959	49.00%	9,344,328	40.52%	2,415,888	10.48%	45	51.72%	34	39.08%	8	9.20%	Conservative	+2.72	Labour	-1.44	Lib Dem	-1.28
Conservative	Labour	Scottish National	434,097	16.72%	707,147	27.24%	1,454,436	56.03%	27	29.35%	25	27.17%	40	43.48%	Conservative	+12.62	Labour	-0.07	Scottish National	-12.55
Conservative	Labour	UKIP	11,299,959	46.07%	9,344,328	38.10%	3,881,129	15.82%	45	53.57%	34	40.48%	5	5.95%	Conservative	+7.50	Labour	+2.38	UKIP	-9.87
Conservative	Lib Dem	UKIP	11,299,959	64.22%	2,415,888	13.73%	3,881,129	22.06%	45	77.59%	8	13.79%	5	8.62%	Conservative	+13.37	Lib Dem	+0.06	UKIP	-13.43
Labour	Lib Dem	UKIP	9,344,328	59.74%	2,415,888	15.45%	3,881,129	24.81%	34	72.34%	8	17.02%	5	10.64%	Labour	+12.60	Lib Dem	+1.58	UKIP	-14.17
Labour	Plaid Cymru	UKIP	552,473	58.87%	181,704	19.36%	204,330	36.98%	46	77.97%	8	13.56%	5	10.66%	Labour	+19.10	Plaid Cymru	-5.80	UKIP	-26.33
Alliance	Social Democrat	Unionist	61,556	17.81%	99,809	28.88%	184,260	53.31%	9.8	18.74%	13.7	26.20%	28.8	55.07%	Alliance	+0.93	Social Democrat	-2.68	Unionist	+1.75
Alliance	Ulster	Unionist	61,556	17.68%	102,361	29.40%	184,260	52.92%	9.8	18.01%	15.8	29.04%	28.8	52.94%	Alliance	+0.34	Ulster	-0.36	Unionist	+0.02
Sinn Féin	Social Democrat	Unionist	176,232	38.29%	99,809	21.68%	184,260	40.03%	27.9	39.63%	13.7	19.46%	28.8	40.91%	Sinn Féin	+1.34	Social Democrat	-2.22	Unionist	+0.88
Sinn Féin	Ulster	Unionist	176,232	38.08%	102,361	22.12%	184,260	39.81%	27.9	38.48%	15.8	21.79%	28.8	39.72%	Sinn Féin	+0.41	Ulster	-0.32	Unionist	-0.09

Which we apply like so:

Constituency	Incumbent			Second Party			Third Party			Modifiers			Prediction
	Party	Votes	3P%	Party	Votes	3P%	Party	Votes	3P%	Incumbent	Second	Third	Party	Vote Share
Aldershot	Conservative	23,369	58.29%	Labour	8,468	21.12%	UKIP	8,253	20.59%	+7.50	+2.38	-9.87	Conservative	65.79%
Ashfield	Labour	19,448	48.35%	Conservative	10,628	26.42%	UKIP	10,150	25.23%	+2.38	+7.50	-9.87	Labour	50.73%
Ashford	Conservative	30,094	58.47%	UKIP	10,798	20.98%	Labour	10,580	20.55%	+7.50	-9.87	+2.38	Conservative	65.97%
Ashton-under-Lyne	Labour	19,366	53.14%	Conservative	8,610	23.63%	UKIP	8,468	23.24%	+2.38	+7.50	-9.87	Labour	55.52%
Belfast South	Social Democrat	9,560	38.36%	Unionist	8,654	34.72%	Alliance	6,711	26.92%	-2.68	+1.75	+0.93	Unionist	36.47%
Birmingham Hodge Hill	Labour	28,069	75.00%	Conservative	4,707	12.58%	UKIP	4,651	12.43%	+2.38	+7.50	-9.87	Labour	77.38%
Blackley & Broughton	Labour	22,982	66.29%	UKIP	6,108	17.62%	Conservative	5,581	16.10%	+2.38	-9.87	+7.50	Labour	68.67%
Blaydon	Labour	22,090	58.45%	UKIP	7,863	20.81%	Conservative	7,838	20.74%	+2.38	-9.87	+7.50	Labour	60.83%
Blyth Valley	Labour	17,813	51.27%	UKIP	8,584	24.71%	Conservative	8,346	24.02%	+2.38	-9.87	+7.50	Labour	53.65%
Bournemouth East	Conservative	22,060	59.77%	Labour	7,448	20.18%	UKIP	7,401	20.05%	+7.50	+2.38	-9.87	Conservative	67.27%
Bournemouth West	Conservative	20,155	57.12%	UKIP	7,745	21.95%	Labour	7,386	20.93%	+7.50	-9.87	+2.38	Conservative	64.62%
Bracknell	Conservative	29,606	63.12%	Labour	8,956	19.10%	UKIP	8,339	17.78%	+7.50	+2.38	-9.87	Conservative	70.62%
Bradford South	Labour	16,328	46.30%	Conservative	9,878	28.01%	UKIP	9,057	25.68%	+2.38	+7.50	-9.87	Labour	48.68%
Braintree	Conservative	27,071	59.07%	UKIP	9,461	20.64%	Labour	9,296	20.28%	+7.50	-9.87	+2.38	Conservative	66.57%
Bridgwater & Somerset West	Conservative	25,020	55.54%	UKIP	10,437	23.17%	Labour	9,589	21.29%	+7.50	-9.87	+2.38	Conservative	63.04%
Broxbourne	Conservative	25,797	59.52%	UKIP	9,074	20.94%	Labour	8,470	19.54%	+7.50	-9.87	+2.38	Conservative	67.02%
Central Devon	Conservative	28,436	66.76%	UKIP	7,171	16.84%	Labour	6,985	16.40%	+7.50	-9.87	+2.38	Conservative	74.26%
Chesham & Amersham	Conservative	31,138	69.09%	UKIP	7,218	16.02%	Labour	6,712	14.89%	+7.50	-9.87	+2.38	Conservative	76.59%
Chesterfield	Labour	21,829	58.08%	Conservative	8,231	21.90%	UKIP	7,523	20.02%	+2.38	+7.50	-9.87	Labour	60.46%
Cynon Valley	Labour	14,532	58.99%	Plaid Cymru	5,126	20.81%	UKIP	4,976	20.20%	+19.10	-5.80	-26.33	Labour	78.09%
Don Valley	Labour	19,621	48.66%	Conservative	10,736	26.63%	UKIP	9,963	24.71%	+2.38	+7.50	-9.87	Labour	51.04%
East Hampshire	Conservative	31,334	72.44%	UKIP	6,187	14.30%	Lib Dem	5,732	13.25%	+13.37	-13.43	+0.06	Conservative	85.81%
Fareham	Conservative	30,689	65.41%	UKIP	8,427	17.96%	Labour	7,800	16.63%	+7.50	-9.87	+2.38	Conservative	72.91%
Grantham and Stamford	Conservative	28,399	60.58%	UKIP	9,410	20.07%	Labour	9,070	19.35%	+7.50	-9.87	+2.38	Conservative	68.08%
Great Grimsby	Labour	13,414	43.69%	Conservative	8,874	28.90%	UKIP	8,417	27.41%	+2.38	+7.50	-9.87	Labour	46.07%
Hackney North and Stoke Newington	Labour	31,357	68.19%	Conservative	7,349	15.98%	Green	7,281	15.83%	-1.40	+3.05	-1.65	Labour	66.79%
Harborough	Conservative	27,675	63.98%	Labour	8,043	18.59%	UKIP	7,539	17.43%	+7.50	+2.38	-9.87	Conservative	71.48%
Huntingdon	Conservative	29,652	60.06%	Labour	10,248	20.76%	UKIP	9,473	19.19%	+7.50	+2.38	-9.87	Conservative	67.56%
Kingston upon Hull North	Labour	18,661	62.77%	UKIP	5,762	19.38%	Conservative	5,306	17.85%	+2.38	-9.87	+7.50	Labour	65.15%
Lagan Valley	Unionist	19,055	62.16%	Ulster	6,055	19.75%	Alliance	5,544	18.09%	+0.02	-0.36	+0.34	Unionist	62.18%
Leeds Central	Labour	24,758	62.47%	Conservative	7,791	19.66%	UKIP	7,082	17.87%	+2.38	+7.50	-9.87	Labour	64.85%
Leeds East	Labour	20,530	57.37%	Conservative	7,997	22.35%	UKIP	7,256	20.28%	+2.38	+7.50	-9.87	Labour	59.75%
Leeds West	Labour	18,456	55.44%	Conservative	7,729	23.22%	UKIP	7,104	21.34%	+2.38	+7.50	-9.87	Labour	57.82%
Ludlow	Conservative	26,093	65.68%	UKIP	7,164	18.03%	Lib Dem	6,469	16.28%	+13.37	-13.43	+0.06	Conservative	79.05%
Manchester, Gorton	Labour	28,187	77.53%	Green	4,108	11.30%	Conservative	4,063	11.17%	-1.40	-1.65	+3.05	Labour	76.13%
Mid Bedfordshire	Conservative	32,544	64.16%	Labour	9,217	18.17%	UKIP	8,966	17.68%	+7.50	+2.38	-9.87	Conservative	71.66%
Mid Norfolk	Conservative	27,206	58.23%	UKIP	9,930	21.25%	Labour	9,585	20.52%	+7.50	-9.87	+2.38	Conservative	65.73%
Neath	Labour	16,270	55.94%	Plaid Cymru	6,722	23.11%	UKIP	6,094	20.95%	+19.10	-5.80	-26.33	Labour	75.04%
Normanton, Pontefract and Castleford	Labour	25,213	56.57%	UKIP	9,785	21.96%	Conservative	9,569	21.47%	+2.38	-9.87	+7.50	Labour	58.95%
North Antrim	Unionist	18,107	63.97%	Sinn Féin	5,143	18.17%	Ulster	5,054	17.86%	-0.09	+0.41	-0.32	Unionist	63.88%
North East Bedfordshire	Conservative	34,891	66.19%	Labour	9,247	17.54%	UKIP	8,579	16.27%	+7.50	+2.38	-9.87	Conservative	73.69%
North East Hampshire	Conservative	35,573	76.47%	Lib Dem	5,657	12.16%	Labour	5,290	11.37%	+2.72	-1.28	-1.44	Conservative	79.19%
North Somerset	Conservative	31,540	66.19%	Labour	8,441	17.71%	UKIP	7,669	16.09%	+7.50	+2.38	-9.87	Conservative	73.69%
North West Hampshire	Conservative	32,052	67.47%	UKIP	8,109	17.07%	Labour	7,342	15.46%	+7.50	-9.87	+2.38	Conservative	74.97%
Ogmore	Labour	18,663	62.83%	Conservative	5,620	18.92%	UKIP	5,420	18.25%	+2.38	+7.50	-9.87	Labour	65.21%
Old Bexley and Sidcup	Conservative	24,682	58.64%	Labour	8,879	21.10%	UKIP	8,528	20.26%	+7.50	+2.38	-9.87	Conservative	66.14%
Oldham West and Royton	Labour	23,630	58.05%	UKIP	8,892	21.84%	Conservative	8,187	20.11%	+2.38	-9.87	+7.50	Labour	60.43%
Orpington	Conservative	28,152	64.03%	UKIP	8,173	18.59%	Labour	7,645	17.39%	+7.50	-9.87	+2.38	Conservative	71.53%
Redcar	Labour	17,946	54.35%	Lib Dem	7,558	22.89%	UKIP	7,516	22.76%	+12.60	+1.58	-14.17	Labour	66.95%
Reigate	Conservative	29,151	68.52%	UKIP	6,817	16.02%	Labour	6,578	15.46%	+7.50	-9.87	+2.38	Conservative	76.02%
Rochdale	Labour	20,961	56.31%	UKIP	8,519	22.89%	Conservative	7,742	20.80%	+2.38	-9.87	+7.50	Labour	58.69%
Romford	Conservative	25,067	53.86%	UKIP	11,208	24.08%	Labour	10,268	22.06%	+7.50	-9.87	+2.38	Conservative	61.36%
Rutland and Melton	Conservative	30,383	64.04%	UKIP	8,678	18.29%	Labour	8,383	17.67%	+7.50	-9.87	+2.38	Conservative	71.54%
South East Cornwall	Conservative	25,516	61.14%	Lib Dem	8,521	20.42%	UKIP	7,698	18.44%	+13.37	+0.06	-13.43	Conservative	74.51%
Sheffield, Heeley	Labour	20,269	58.96%	UKIP	7,315	21.28%	Conservative	6,792	19.76%	+2.38	-9.87	+7.50	Labour	61.34%
Sleaford and North Hykeham	Conservative	34,805	63.04%	Labour	10,690	19.36%	UKIP	9,716	17.60%	+7.50	+2.38	-9.87	Conservative	70.54%
South Basildon and East Thurrock	Conservative	19,788	45.62%	UKIP	12,097	27.89%	Labour	11,493	26.49%	+7.50	-9.87	+2.38	Conservative	53.12%
South Staffordshire	Conservative	29,478	62.92%	Labour	9,107	19.44%	UKIP	8,267	17.64%	+7.50	+2.38	-9.87	Conservative	70.42%
Southend West	Conservative	22,175	58.15%	Labour	8,154	21.38%	UKIP	7,803	20.46%	+7.50	+2.38	-9.87	Conservative	65.65%
Stirling	Scottish National	23,783	48.40%	Labour	13,303	27.07%	Conservative	12,051	24.53%	-12.55	-0.07	+12.62	Conservative	37.15%
Stoke-on-Trent Central	Labour	12,220	46.52%	UKIP	7,041	26.80%	Conservative	7,008	26.68%	+2.38	-9.87	+7.50	Labour	48.90%
Stoke-on-Trent North	Labour	15,429	43.38%	Conservative	10,593	29.79%	UKIP	9,542	26.83%	+2.38	+7.50	-9.87	Labour	45.76%
Strangford	Unionist	15,053	61.17%	Ulster	4,868	19.78%	Alliance	4,687	19.05%	+0.02	-0.36	+0.34	Unionist	61.19%
Stratford-on-Avon	Conservative	29,674	68.77%	UKIP	6,798	15.75%	Labour	6,677	15.47%	+7.50	-9.87	+2.38	Conservative	76.27%
South West Surrey	Conservative	28,784	72.25%	UKIP	5,643	14.16%	Labour	5,415	13.59%	+7.50	-9.87	+2.38	Conservative	79.75%
Tewkesbury	Conservative	30,176	65.59%	Labour	8,204	17.83%	Lib Dem	7,629	16.58%	+2.72	-1.44	-1.28	Conservative	68.31%
Thirsk and Malton	Conservative	27,545	63.41%	Labour	8,089	18.62%	UKIP	7,805	17.97%	+7.50	+2.38	-9.87	Conservative	70.91%
Thurrock	Conservative	16,692	34.37%	Labour	16,156	33.27%	UKIP	15,718	32.36%	+7.50	+2.38	-9.87	Conservative	41.87%
Tonbridge and Malling	Conservative	31,887	66.93%	UKIP	8,153	17.11%	Labour	7,604	15.96%	+7.50	-9.87	+2.38	Conservative	74.43%
Truro and Falmouth	Conservative	22,681	57.90%	Lib Dem	8,681	22.16%	Labour	7,814	19.95%	+2.72	-1.28	-1.44	Conservative	60.62%
Washington and Sunderland West	Labour	20,478	58.79%	UKIP	7,321	21.02%	Conservative	7,033	20.19%	+2.38	-9.87	+7.50	Labour	61.17%
Wellingborough	Conservative	26,265	57.13%	UKIP	9,868	21.47%	Labour	9,839	21.40%	+7.50	-9.87	+2.38	Conservative	64.63%
West Bromwich West	Labour	16,578	49.08%	UKIP	8,836	26.16%	Conservative	8,365	24.76%	+2.38	-9.87	+7.50	Labour	51.46%
West Tyrone	Sinn Féin	16,807	56.03%	Unionist	6,747	22.49%	Social Democrat	6,444	21.48%	+1.34	+0.88	-2.22	Sinn Féin	57.37%
West Worcestershire	Conservative	30,342	66.91%	UKIP	7,764	17.12%	Labour	7,244	15.97%	+7.50	-9.87	+2.38	Conservative	74.41%
Weston-Super-Mare	Conservative	25,203	57.07%	Labour	9,594	21.72%	UKIP	9,366	21.21%	+7.50	+2.38	-9.87	Conservative	64.57%
Wigan	Labour	23,625	56.48%	Conservative	9,389	22.44%	UKIP	8,818	21.08%	+2.38	+7.50	-9.87	Labour	58.86%
Witham	Conservative	27,123	64.34%	UKIP	7,569	17.95%	Labour	7,467	17.71%	+7.50	-9.87	+2.38	Conservative	71.84%
Wokingham	Conservative	32,329	67.31%	Labour	8,132	16.93%	Lib Dem	7,572	15.76%	+2.72	-1.44	-1.28	Conservative	70.03%
Wolverhampton South East	Labour	18,539	55.57%	Conservative	7,761	23.26%	UKIP	7,061	21.17%	+2.38	+7.50	-9.87	Labour	57.95%

and that completes our predictions:

That amounts to the Conservatives winning 23 seats (14 from Labour, 7 from the Scottish Nationals, 1 from the Lib Dems and 1 from UKIP), Labour winning two (1 each from Plaid Cymru and the Scottish Nationals), Sinn Féin winning one from the Ulster Unionists and the Democratic Unionists taking one from the Social Democrats to give:

Conservative: 353 (Majority)
Labour: 220
Scottish Nationals: 48
Democratic Unionists: 9
Lib Dems: 7
Sinn Féin: 5
Plaid Cymru: 2
Social Democrat: 2
Green: 1
Ulster Unionists: 1
Independent: 1
and the Speaker.

As a completely unjustified footnote, however, I expect the Scottish Nationals to do better here than expected, based on a surge in nationalism and a desire for a second referendum to leave Britain and rejoin the EU. I expect UKIP to lose more than their one predicted loss now that their main draw card—leaving the EU—has been achieved. And I expect the Lib Dems to do better than suggested, returning to their default position of 3rd party nationally after an anomalously bad result in 2015 skewing the data.