Also, though they're not sitcoms at all, these need always be mentioned in discussions of UK comedy:
Would I Lie To You?
Never Mind The Buzzcocks
Mock The Week
8 Out of 10 Cats
The Big Fat Quizzes
Mock The Week is probably one of my favourite shows ever. I like to call seasons 3-6 the "golden era" of that show, I miss Frankie and Russell.
NMTB is great too, though I haven't watched it in a while. I find some of the hosts are just awful, I wish Simon Amstell was still doing it.