I wouldn't say so. From my time out on exchange, at Idaho State, I would say that pretty much everyone that I encountered from Idaho, Colorado, Utah and Montana, as well as from Virginia, were very hosipitable, genuine and sincere, always giving me the time of day, always willing to hear me out. Ultimately would there be people in the UK willing to do the same for me? I don't know, but I would hope it is so.
In terms of politeness, everyone I came across except for one Californian and New Yorker were exceptionally polite. My time in Atlanta was pretty positive, although my time in D.C. and Chicago, I would say less so, in terms of politeness. One thing I would say, is, I know I was fairly sheltered, what with the novelty factor of being an exchange student.
That said, I still think that politeness and sincerity are not things that could really be stereotyped. It depends on one's upbringing to be honest. There's a lot of polite and sincere British people, just as there are a lot of insincere and unpolite American people.