A comparison list compiled by tripadvisor on city costs:
what are people's thoughts?
lucky you can afford taxis in london jerrycoin
I guess the list in that article just confirms what I guess most people already knew.
As for taxi's in London, I don't bother (except if work are paying). I walk, cycle, or use my Oyster card on the tube / bus. Oh, always use an Oyster card, never buy a day travel card.