Brexit has been massively divisive, and was the beginning of the train of short-term PMs we've had over the last 10 years. I don't see how another issue like that would (or could) serve as a unifier.

There are various factors at play, some of which are by no means unique to the UK like the rise of anti-immigrant sentiment. But IMO one aspect which is somewhat UK-specific is the first past the post system, which doesn't really work that well in situations where there are more than two parties seriously challenging for power. Starmer won in a "landslide" when you look at seats gained, but actually a lot of that was due to the right wing vote being split between the Tories and Reform. And on the other side you have left or centre-left parties like the Greens and Lib Dems which are now threatening to split the left wing vote. It's a pretty nonsensical and undemocratic system and I do think it plays a large part in the fact that no government ever seems very safe, even when they get a large surplus of seats in Parliament.

(I am not a UK citizen but I have lived here for years.)