MOnica, You're not taking into account the complete collapse of the Conservative Party, most of whose former voters almost certainly have defected to Reform. Labour's support has dropped without any doubt and possibly some has gone to Reform (but more likely Greens or LibDems). What that means in a FPTP system is that Reform has ended up the biggest party in some councils/constituencies.
Prof Sir John Curtice has shown that the Greens are doing more damage to Labour than Reform. I can't find a link which isn't embedded into a Facebook or X post, but you can find an interview with him by Googling "Professor John Curtice says that voting shows the Green Party are doing more damage to the Labour Party than Reform".
It's a strange situation. Whereas the majority of voters have opinions somewhere in the centre, the FPTP system means that the extreme parties are winning seats by achieving majorities because the opposition is splintered. During a general election, there will be more tactical voting. People will be more likely to come together to keep certain candidates out.