STV would be ideal, but there's a large contingent of Labour centrists who are happy with FPTP precisely because the think it gives them the right to demand the votes of people who would naturally vote for other parties.
If RefUK win today there's going to be a wave of insufferable, holier-then-thou centrists complaining about the Greens splitting the vote.
.....if the Greens win otoh then Labour might need to seriously consider switching to PR before the next GE, because FPTP can be brutal in heavily-split, multi-party elections and if voters are leaning towards RefUK/Greens rather than Tory/Labour then they could easily be wiped out like they were in Scotland.
Under PR they'd survive.
I'd like to believe that the more pragmatic Labour supporters would see the writing on the wall and flip to the Greens, but I strongly suspect that if RefUK win then they'll whinge abut Green vote-splitting regardless of who comes second and third.