No one who matters ever doubted that.
BUT the UK voters are quite reluctant to have them for govt and usually give up on them after just one term. Blair/Brown are the only ones who did better than that and they were only Labour in name only.
And given that Starmer is so enthustiastically kicking the previous left leaders like Corbyn and Abbott right out of the party, it remains to be seen what Labour voters will make of that. He's even sanctioned one of his shadow ministers for daring to show up at a strike picket line.
Otoh with the SNP imploding so spectacularly it remains to be seen what that will do for the Labour vote in Scotland.