I know you're just kidding, but I wanted it on record that all the people we met during our time in London and Cambridge were friendly and nice. I didn't get those antisocial impressions at all.

I don't think it's really viewed as anti-social. Brits (in general, not universally) don't strike up conversations with other Brits because they don't much like other Brits striking up conversations with them. Most of those same Brits, though, will behave differently to foreigners, cheerfully engaging them in conversations. I don't know whether this is because we think foreigners (and particularly Americans) naturally appreciate a nice chat with a complete stranger more than Brits do, or because we think people visiting foreign countries will appreciate being put at ease or offered advice or assistance... actually it must be the latter, because the only time you're guaranteed to see British strangers engaging each other in conversation is when both are overseas...

Peter