By calculating how many seats are safe in the UK, we can try and plot a course for a Progressive Alliance (broadly, parties which are anti-hard Brexit) to prevent a Conservative majority.
We have previous looked at England (north and south), and now we look at the rest of the UK.
It is in Scotland, Wales and Northern Ireland where a Progressive Alliance has a particular difficulty, as nationalism means that parties are divided on an issue as big as Brexit. We will look at the individual politics of each region in turn.
With the Greens and UKIP having little chance of taking a seat in Scotland (in fact, a 0% chance in our model), this leaves the seats to be fought out between the SNP, the Conservatives, Labour and the Liberal Democrats. All but the Conservatives will oppose a hard Brexit, but as Labour and Lib Dems are against Scottish independence, a formal pact between these parties and the SNP is vastly unlikely. Therefore, this guide is more for voters of these parties, for whom a hard Brexit is the main thing to avoid.
Total seats: 27
Safe seats: SNP 19
Total seats: 32
Safe seats: SNP 7
For Labour, they currently are favourites to keep Edinburgh South, and Labour activists in the region should campaign there; if they were to campaign in Edinburgh North & Leith or Edinburgh South West, they would risk giving a seat to the Conservatives. Labour voters in East Lothian may be crucial to the outcome of that seat, as Labour are fairly strong in this seat, but not enough to have a great chance of winning.
Meanwhile, for the Liberal Democrats they are also best focusing on the aforementioned seats they may win, plus Ross, Skye & Lochaber where there is little Conservative threat. However, for the Lib Dems, their voters may be more likely to vote for the Conservatives than the SNP (due to the nationalism divide), therefore unlike Labour, they might be valuable in helping the SNP in SNP-Conservative marginal seats if they have a strong performance.
In Wales there is a larger UKIP presence than in Scotland, with several UKIP members elected to the Welsh Assembly last year. With no Green presence in the Assembly, and little predicted Green chance across Welsh seats, that leaves three anti-hard Brexit parties - Labour, the Lib Dems and Plaid Cymru. As Plaid are a nationalist party, Labour and the Lib Dems will be hesitant about any formal alliances. Therefore, as with Scotland any tactics involving Plaid are more likely to apply to voters rather than the parties themselves.
Total seats: 40
Safe seats: CON 6; LAB 3
For the nationalist Plaid Cymru, they are currently on three seats, all of which they will likely retain, whilst they have a very strong chance of taking Ynys Môn, and a good chance of taking Rhondda, both from Labour. They also have outside chances in Ceredigion, Blaenau Gwent and Cynon Valley, none of which have large Conservative support. Plaid voters in other seats may be loathe to vote for a unionist party, however if they oppose a hard Brexit then it might be in their interests to do so.
The Lib Dems appear likely to retain Ceredigion, and can therefore focus on winning additional seats. Their best chance is in Cardiff Central, followed by Brecon & Radnorshire. However, they may be inclined to focus on the latter, as this seat is currently held by the Conservatives, and winning this will do more for the Lib Dems' agenda than winning the Labour-held Cardiff Central.
Northern Irish politics is significantly different to the rest of the UK, with entirely different parties. Of course, the main issue is between unionist and republican parties and it is unfeasible that parties on different sides of this issue will work together. However, Alliance are neither unionist nor republican, and activists of theirs may be open to voting tactically. As it happens, the two main unionist parties, the Democratic Unionists and the Ulster Unionists, are pro-Brexit, and are likely to support the Conservatives, whilst the SDLP, a republican party, are against a hard Brexit. Meanwhile, Sinn Féin, the other republican party, do not take seats in Westminster and therefore count as neutral in terms of a Progressive Alliance. Finally, there is one independent candidate, Lady Hermon, who is a unionist who is against a hard Brexit.
Therefore, the aim of a Progressive Alliance, would be for as many SDLP and Alliance seats (plus Lady Hermon). If they can't achieve that, then they would prefer Sinn Féin to take seats ahead of the DUP and UUP.
Total seats: 18
Safe seats: DUP 5; SIN 4; IND 1
The SDLP currently hold Belfast South, Foyle and South Down, and currently are precariously placed to keep all three. In all three, the SDLP would greatly benefit from voters who might otherwise be inclined towards another party. In particular, Alliance voters could make the difference in Belfast South. In return for any help in Belfast South, SDLP voters may be inclined to vote for Alliance in Belfast East, where there is a strong chance they can take the seat from the DUP.
As Sinn Féin is opposed to Brexit, they may be more inclined to push resources and activists towards Belfast North and Fermanagh & South Tyrone, rather than the SDLP-held seats.
Whilst politics in England are relatively straightforward, politics elsewhere is far more nuanced, especially due to the influence of nationalism. This means that whilst there are several opportunities for tactical voting to be used to prevent votes for pro-Brexit parties, some lines are impossible for voters to cross. However, for many, Brexit is a hugely critical issue and this article provides a guide of how to vote in order to achieve the desired result.