Your best bet is to pick up an *UNLOCKED* Triband GSM phone with a 110/220V power supply in the states (most Sony Ericsson phones work well for this--find them on eBay) and a Virgin Mobile SIM in the United Kingdon. While in the UK, inbound calls are free, I believe it's 40p for outbound to the US and roaming in the EU is 99p inbound or outbound. The SIM costs £10 and includes £5 of credit. It can be purchased at any of the six trillion mobile phone stores on the High Street or at The Link in T3 in LHR.
Timothy