If you go to the UK quite a bit and you want to have a consistent UK number, you may want to investigate the Virgin Mobile option. The joining pack (with SIM card) is 10pds (with 5pds of airtime) and the value of the SIM never expires i.e. the number is yours to keep for life. I'm not sure if other UK mobile providers have the same lifetime option but most pre-paid SIM cards expire after a certain period of time (usually 90-180 days).

With a lifetime SIM, the next time you go back to the UK, just top up the value of the SIM and start calling. Incoming calls are free, calls back to the US are 20p/min. Make sure your phone is unlocked! I don't know about Cingular but I know it's very hard to get unlock codes from AT&T and T-Mobile will give it to you after 90 days.