There aren't too many good examples that I can think of, and regional accents are so diverse here that I'd be as poor a judge of say a Cumbrian accent as most of you would be. Generally people like Paltrow manage a successful modern English, somewhat south-eastern accent. The kind of relatively neutral accent that newscasters would have if the BBC weren't so obsessed with regional accents.
I'm not always the best judge. For instance House still sounds like Hugh Laurie to me, I know he is doing an accent and I can tell he is doing it well, but it is only when I contrast a youtube clip of House with an interview that I can really hear it. I can tell the differences between the most general sorts of American accent, but I've offended Canadians before. I watch a Canadian show called Durham County, and they might as well be Americans for all I know. I was somewhat relieved to see Americans comment that Sarah Palin sounds sort of Minnesotan and they sound sort of Canadian.
Others attempting particular British regional accents is pretty much asking for trouble. Think **** van **** in Mary Poppins. Adam on Mythbusters does a funny sort of London/Essex accent, that makes me laugh in part because it's not quite right and he knows it.