When I was in UK I purchased a SIM card from Virgin Mobile and it worked very well all over. I understand the rates are very comparable. I found it to be pretty inexpensive and you do not have to purchase minutes every 30 or 60 days like in the USA, usually all you have to do is fill it up once a year.
I believe calls are about US 0.15 cents a minute and are billed in fractions of a minute. Also my calls to the USA only cost me about US 0.25 - 0.30 cents a minute.
FYI- I still have my Virgin Mobile SIM card active. If you want I can sell it to you this way you have a telephone number before you leave. There is also some minutes left in it, maybe enough for about 10 minutes of calls. As soon as you get there you can fill it up. I purchased this SIM card in December of 2004, so as long as you get there before December it should be fine. If interested let me know.