The GSM phones have a SIM card with all your customer details. If you get a US GSM SIM you can put it in the (triband) mobile when there and keep the UK one for Europe. Some of the UK operators have some deals on international tariffs, but they are all steep. The US GSM is PCS (1900 MHz) also on PCS is TDMA and CDMA, (they are both on 800MHz ‘Cellular’as well). The big operators of TDMA (IS136) such as AT&T are converting over to GSM or 2 and a half G (GSM with GPRS), they will eventually be on the lower frequencies as well.
Or you could get a WIFI phone using Voice over IP no international call charges, and Airports and Hotels will be the first to get coverage.
CC