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|
|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|
|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|
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: