pounds

So, I’m planning to travel in the winter to Europe and I wanna spend two weeks at friend’s houses in London (in january/february 2010). For the others countries I’m planing 35-40 euros per day (without accomodations and transportation beetween cities) but I have ANY idea how much money is the ideal per day in London. I know it’s a really expensive city and how much in euros (or US dollars) do you guys think it’s necessary? (considering I’ll have a really short budget)
thank you and sorry for my bad English (I’m brazilian)