I will be in London for a short time at the end of June, and can't live without a cell phone. I have an unlocked GSM phone, and could buy a prepaid SIM card here, but it looks like it calls would cost a lot less if I got one there--could someone explain what to look for and how it works?

I had a very bad experience "renting" a card in France last summer, had an unexpected $1000 "deposit" charged to my AmEx that took six months to get bac, and ended up being billed about three hundred dollars more than I should have--so I am a bit wary--