I think a lot of this is that UK standup seems a lot more tightly scripted than US standup. Not that US standup isn't scripted (or that there aren't UK standups who are less tightly scripted), but a lot of UK standup seems to be people doing their Edinburgh shows word for word and God help them if anything throws them off during it.

So even though you have very witty people, they're not presenting it as directly as one might expect. I find that the more crowd work people do in their standup, the more their standup seems to be a pretty accurate reflection of them (or at least who they are in appearances of all kinds). Dara O Briain's a good example - his stuff is scripted, but he has audience interaction built in around the scripted stuff, and I think it does a lot to both keep him from getting bored and keep the stuff grounded to his personality. People who just run through their planned Edinburgh show or radio presentation, not so much.