Nah, cost of living isn't that high (as ig1 says). I've been living in London for the last 7 years, much of it on a rather limited salary. You can live very comfortably near central London (zone 1-2) in a nice area on £35k. You won't save much money, but once your salary goes up, of course, you will be able to (depending on how you adjust your lifestyle, of course).
As for hourly rate, that can be computed easily. My friend works about 12 hours a day, 5 days a week (no weekends, since her work is linked to the markets being opened). She gets 4 weeks of holiday a year, and there's about a week's worth of bank holidays. So that's 47 weeks * 60 hours = 2820 hours a year, on the upper end (assuming no sick days or other unplanned absences).
That works just above £53 an hour.
You could drive that down by counting her (30 minute or so) commute in, but you won't make much difference. The fact is, she earns very good money no matter which way you cut it. And that's not even counting the (very significant) bonus.
I'll grant you that living in Cambridge and working in London is daft, but that's why they have, ya'know, houses and stuff in London. So you can move there rather than spend hours on the train every day.