You can buy a Virgin phone for about $60. These are pay as you go phones--you buy cards with prepaid time. They have pretty incredible rates, After paying 15 pence/min for the first five minutes of the day, additional time is 5 pence/min. Calls back to the US are 20 pence/min. Plus you can keep the phone for future trips to Britain. They will also provide the unlock code for free so that you can use it in other countries.