As others have mentioned, I don't think Labour will do well in the next election based on what they've been doing so far. So voting for "third" parties is inevitable. It's just a question of whether we can get a majority of progressives, or we just end up spread out and Reform sweeps the win.
I think maybe the ideal solution would be if the progressive parties formed some sort of coalition ahead of time, centred around electoral reform. Discuss who contests which seats, etc, and how they'd vote as a coalition.
Well the most ideal would be if they did that with the express purpose of getting electoral reform done and then calling a general election right after. That way even conservatives and right wingers might vote for them.
It really is past the time when Britain finally went through some actual changes.