Well, I was one of the 'freakishly thin' people living in London, and yes, the accidental exercise of walking everywhere, or getting public transport that's a 20minute walk to the station, 10-20 min walk at the other end done 2x a day, and not having a car with you if you need to nip to the shops in your lunch break so walking to the tube again. Being constantly late so running up the stairs (an ex-colleague of mine always said, "the right of the escalator is for the eldery, the sick,the pregnant, and the hungover.").
But more, it's the 'hurd' mentality towards weight. When you're surrounded by slim people, being overweight, even by a little, is more obvious. I know I felt fine as a large size 8 on mat leave after having DC1, compared to many of my 'mummy' friends I'd met out here in the Home Counties (moved out when pregnant like many middle class 30-something Londoners!), I was looking ok. When I started getting the train back into London I suddenly felt fat, and the last couple of Kgs just went.
It's easier to eat healthier if you are surrounded by people making an effort about their diet, it's a lot harder if you're surrounded by people not doing so, if you are used to seeing a wobbly mummy tummy on all the other woman you see day to day, then it's not a big deal you have one, if you are used to seeing slim woman, then you will feel bigger and more likely to be inspired to lose it.
Plus there's the 'achiever' attitude, London is full of people who push themselves, it often shows via careers, but driven people are rarely only driven in one area, that attitude towards education, careers, fashion, social life also gets targetted on their own bodies.
(and yes, it is possible to be slim and be a high achiever and many other things in other parts of the country, this is just why they concentrated in London)