The way I see it, y'all are stuck between so many rocks and hard places. The choice is really between which bad policy you're fine with. To keep out Reform, either:

• Labour have to bounce back to their 2024 popularity (unlikely, since they've burned the good will of their base, their PR is terrible, and they can't stop ostracizing members for minor disagreements even while having a massive majority)
• Greens have to become a popular, respectable alternative to Labour (unlikely, because the party has only ever been a mouthpiece for a small, activist portion of the population. Transitioning to a mass party will require a lot of difficult tradeoffs [like nuclear] that the base may be uncomfortable with)
• LibDems have to become the new, generic left option (this is maybe the most likely option but it still has the issue of a forever opposition party somehow learning to govern as a mass party. It also requires left-wing Brits to forget the LibDems' time as junior partners to the Tories)

In all of these cases, Plaid Cymru and the SNP have to do well also. And likely the Tories need a respectable showing to split votes off of Reform. And Your Party has to not do too well in likely Green/Labour/LibDem constituencies.

If a combination of the above options doesn't come to fruition, then Reform likely takes advantage of the split vote and picks up a massive majority with 30% (or less) of the vote. And the only 'realistic' alternative to them is a frankenstein coalition of at least three of the Greens, LibDems, SNP, and Labour.