London is sick but mind the exchange rate and expect to ball out. I just got finished doing something similar to what you are discribing and with around the same budget. that said I think I spent more money in London in 2 days than I spent in croatia in a week. london also serves as a good jump off point to a lot of other cities in europe because of the budget airlines that fly out of there namely ryan air and easy jet. do be careful when flying these guys though because they find ways to rape you if you don't know how to use them properly. That you have a free place to stay so close to downtown should be a huge leg up.
You can work stupid, but you can't fix a fat body.