Let me quote from For News censured for rant at BBC (link seen on Neil Gaiman's blog.
This is a tricky issue for Ofcom: how to regulate channels which are not produced principally for viewers in Britain. The Independent Television Commission, which preceded Ofcom, responded to complaints last year that Fox did not meet its strict "due impartiality" rules by issuing a ruling that is regarded in some quarters as a fudge to avoid a standoff with Mr Murdoch: it said "due" meant "adequate or appropriate", and Fox News could justifiably claim to have achieved a level of accuracy and impartiality that was appropriate to its audience in the US, where different rules apply.
The truth is such a slippery thing that I don't know if I would want such regulation in the States; however the idea of Fox News being punished for its slander is mighty appealing.