I'm from Exeter and I don't meet many polite people if I'm honest. Most people are either arrogant rich brats or aggresive horrible chavs. The elderly (the supposed polite generation) are rude, racist, self-righteous and seem to spend most of their time 'accidentally' hitting people with their shopping bags and tutting under their breath.
On the other hand, I try and be polite to everyone. I've lived in Brighton and that's a lovely city. I can never get the hang of London, though.
So - I think it depends on where you are but I do think the politeness of Britain is a myth, or at least it's disappearing fast. Every American I have talked to or met (discounting Xbox live for obvious reasons) has been very lovely, the same true of Canadians, French, Finnish, Danish, yada yada.