It was a landslide victory for the labour party. I think the reason why is because the Reform party siphoned off some of the conservative votes. If the Reform party wasn't there, they would have voted conservative because the goals of the conservative party and the reform party are the same. To stop illegal people trafficking on the English channel and bring down some of the taxes to make Britain more competitive globally.
The newly formed reform party acted like a wrecking ball and smashed some of the conservative votes. They split the center-right votes into two smaller groups. That is my opinion though, it may not be the truth. There were other things that may have brought down the conservatives, I remember the frivolous spending during the pandemic and the rule breaking in the cabinet. I won't name anyone but it happened too often and annoyed many ordinary people.