I've heard "yeah well, your mum" in mock arguments many times but hardly ever a full "your mum" joke/insult. (the south of England here, but mainly at university, quite possibly from some north Londoners)
I've also seen some Britishers (ok, Scottishers) have a full "yer maw" joke fight on the internet but have no idea whether they'd do it out loud.
― a passing spacecadet, Wednesday, 21 November 2007 21:18 (eighteen years ago)