If you spend a lot of time in the U.K., buy a Virgin prepaid card when you get there (from any Virgin Superstore and others). For GBP12 you get GBP10 credit, expiration a year (or never? unclear). Calls to U.S. are 15 pence, if I remember correctly. Of course your friends at home will have to learn your new number (unless you have some kind of forwarding at home). The network that Virgin uses is only 1800 MHz, though.

Non-Powertel customers who travel outside the U.K. might be interested in Swisscom EasyRoam, a prepaid with pretty good global roaming agreements. It uses a callback system to keep track of your remaining credit, and you can't originate sms on it, but calls tend to be $1.50 outgoing, 0.70 incoming anywhere. Sometimes the local carrier adds a charge on top of that. EasyRoam has a one-year expiry.