I'd say UK can be either way, but it'd be mostly affectionate between friends, but most always offensive to a stranger.
I'd say 'daft cunt', or 'stupid cunt' , 'cheeky cunt' said to the face is much more likely to be genial mockery. 'useless cunt' is probably mildy offensive but could be either way.
'fucking cunt', or any of the above with the word fucking in there - more likely to be offence intended.
and generally you'd not say it to a stranger unless you're - varies regionally and by dialect of course. and as for 'posh cunt', 'tory cunt', 'bluenose cunt' . . . pretty much, offence intended.
Julian Cope has a story about his song, "cunts can fuck off" where he was helped out by a rural Irish Farmer who told him "hop on the back ya cunt" . Of course that might just be the Gaelic word for "Englishman".
"Serving cunt" (attractive, bold, confident, Bayonetta) and "cunty" (hello kitty, pink, barbie-core) are two recent favorites.
Actually... no, those aren't the same thing. They're a little bit similar.
