I've done some reading and it turns out that Reform is now sometimes polling at a percentage equal to what Labour last won the elections with (~34%). Labour is polling as low as ~20%, the greens at ~10%. So yes, Reform and Tories are splitting the rightwing vote, but no, the left cannot afford to further split the left vote.

Because of fptp, that 34% result was enough to bring Labour to a 63% majority. Which apart from being ridiculously unrepresentative, also means that Reform could achieve the same result.

As an external observer who would rather not have Reform get in control of the UK, I see 2 possible solutions:

1. Get rid of fptp asap.
2. If that's not possible for reasons, then coordinate in between moderate parties to let the top moderate candidate run unopposed against Reform, the French way.