In the last election, 2/3 of the votes Tories lost went to Reform UK, Farrage's fascist party, which gained twice as many new voters as Labour, Lib Dems, and Greens combined. Or to put it another way, Labour votes went up by 5%, Lib Dem by 6%, Green by 150%, and Reform by 615%:

https://www.bbc.co.uk/news/election/2024/uk/results

While Reform only won 5 seats in the Parliament due to the "first by the post" system, it came third in terms of actual vote share, way above Lib Dems. If UK had proportional representation, Reform would now hold 93 out of 650 seats.

What I'm trying to say is, if there is another election, Labour will lose at least as much as Tories lost last time given their unprecendented drop in popularity (https://www.politico.eu/article/uk-keir-starmer-labour-suffers-historic-drop-in-approval-ratings/), but those votes won't go to Tories. Some might go to Greens but most will go to the fascists. And this time, they will have enough to be "first by the post".

I guess one can hope Corbyn manages to organise people like Melenchon did in France but honestly I don't see this happening in the UK cause the public is extremely docile and reactionary. Not that it did the Frenchies much good anyway, the anti-war coalition won and France just went to war with Russia: https://www.rt.com/news/608106-france-ukraine-permission-missiles/