The US supplanted the UK as the de facto leader of the imperial core after WW2, and specially after the Suez crisis.

The UK can't afford to antagonize the world by directly voting against international will anymore like the US now can, hence them (and Switzerland) only abstaining.

The British people aren't that pro-imperialist in general anymore and, while the current government very much IS, I'm pretty sure that it's almost election time, so they have to be careful about being too honest about it right now..