One impediment in the way of widespread Remain Alliance pacts is "short money" i.e. the money that opposition parties receive. It can be a significant source of funding for small parties. Greens got £112,888.78 in 2018/2019 and Plaid Cymru £101,920.04.
researchbriefings.parliament.uk/ResearchBriefi…
researchbriefings.parliament.uk/ResearchBriefi…
Snag is, it's based on a combination of total votes received and seats won. You have to win at least 1 seat and 150,000 votes across the UK, or 2 seats and no minimum vote total. So if candidates don't stand in dozens of constituencies because of a pact, the vote total plummets.
Now, it's all very well to say "country before party" because that's fine in the abstract. But it is a real consideration that can't just be swept under the carpet. Every extra 1,000 votes a party can muster up nationally will be worth £176.5 a year in short funding.
(Intriguingly, although it's a very broad analogy, that would also suggest that your vote has a notional value of £0.1765 a year which over a 5 year parliament comes to a bit less than a quid.)
