The answer?
SNP
Plaid Cymru
The Conservatives
Labour
Liberal Democrats
Greens
Brexit Party
The problem is in a two or three way marginal, the danger is if the pro-Remain or pro-Brexit vote is split, the other side wins
Tory held, Labour 2nd, Lib Dems far back in 3rd
Danger is Lib Dems and Labour split the Remain vote, so the Tory wins
Tory held, Lib Dems best placed to come 2nd or win, Labour likely 3rd
Danger is Lib Dems and Labour split the Remain vote, so the Tory wins
Labour very narrowly held, Tories 2nd, Greens campaigning energetically with a good candidate
Danger is Greens take Labour votes, so the Tory slips through and wins
Labour held, Tories 2nd 2017 but UKIP second 2015. Brexit Party did not stand down here
So pro-Brexit vote is split, meaning Labour can slip through and win the seat
In Wales, Plaid Cymru have struck deals with the Liberal Democrats to avoid those two parties splitting the vote, and it's pretty sure the Plaid vote will not let in a Tory anywhere
And the Conservatives are the best placed pro-Brexit Party in every constituency, so it makes no sense to stand down
And yes, I know Labour people dislike Lib Dems and vice versa.
And yes, I know the UK's First Past the Post election system is grim.
Labour in Putney
Lib Dem in Wokingham
Labour in Stround
Labour in Hartlepool
... and many others I have explained here: tacticalvoting.jonworth.eu
/ends