Realpolitik perspective defeating the nazis still feels like a good long term investment for Britain (though admittedly at enormous cost). We lost our empire, but we prevented a pan Europe empire forming right next to us. A united Europe with an expansionist bent would be the biggest threat to Britain I can imagine. Even if they left us alone for a while they’d surpass in capability once they had a chance to rebuild from the war, and at some point they’d start pushing their weight around, and we’d lose our empire then presumably they’d gobble us up on their way to a squabble with the US or something.

We won ww2 to keep Europe separated.

From the same perspective, brexit was a real blunder: a united Europe and we’re sat on the sidelines waiting to see what they do next? And now the Russians are forcing them to arm up and work together, we’ve practically recreated the lose scenario of ww2, admittedly with less nazis (which I admit is a pretty major win).

Edit to repeat: this is the realpolitik perspective, my real view is that the empire loss was a win for the world in general, the Nazis were a threat to humanity in general and that the EU is a really good neighbour to have, but it’s fun to look at things from the Machiavellian perspective occasionally.