Agree with the above poster - I was in London last week and walked into a T-Mobile store. Asked for a new SIM card with data and international calls.
They gave me a new SIM for free, and charged me 15 Pounds for 1GB internet for one month, and cheap international calls.
Phone plans are crazy cheap in Europe - I travel to different countries often for business, and always pick up a local SIM card. It's much cheaper than roaming charges from back home!