Oh, I do speak to strangers, even in London.
That being said, people comment on it - at our local childcare co-op, about half the parents have come to the co-op via me, and one claims we met because I walked up to her in a park and started talking to her . I don't remember doing this, but it wouldn't be out of character, so I probably did do it.
DH was in a park in Canada in the summer with one of my friends, who was explaining that he makes his own aftershave now. The woman next to them got involved in the conversation, and got his recipe, because she was interested. DH pointed out, that would never happen in the UK. He's probably right.
(But then again, there's a dark side to being overly friendly, some North American women (not me!) will tell other women they don't know about their periods, appropros of basically nothing. So I'd rather have UK standoffishness ... as long as I get my 'foreigner exemption', so I can do what I like.)