So how the frick is this going to work with a General Election? FPTP isn't built for 5 party politics.
I would assume the following will occur:-
Change UK and the Lib Dems will effectively merge. (Change UK couldn't get a seat in a PR system, they have no chance in a FPTP system, other than *maybe* holding a couple of their current seats.).
The Brexit Party will tactically select its candidates. Anywhere that there is a Pro-Brexit Tory candidate that might lose in a split vote, they won't put a candidate forward.
The new Lib Dems and the Greens will each avoid putting forward candidates in a handful of their marginal seats. Because the Lib Dems and the Greens are so different in nearly every other area, it's politically difficult - and a (remain leaning) Lib Dem vote is probably as likely to vote Labour as their second preference than a Green. Likewise, a (remain leaning) Green voter is probably as likely to vote Labour or a smaller party. But, although there won't be a remain alliance, there will be a handful of seat selection pacts.
Labour will still be trying to be all things to all people