For those of you thinking about bringing an unlocked mobile (cell)phone to the UK and buying a sim card. This looks like a good PAYG package to buy. One of the bonus options is a set amount, varies according to the prepay you load, of free minutes to call North America and other countries.
http://www.talkmobile.co.uk/pay_as_you_go.html
You can buy it from the, almost everywhere, Car Phone Warehouse stores.
If you do not have a unlocked phone they also sell very cheap unlocked handsets when bought with a PAYG calling plan.
http://www.carphonewarehouse.com/mobiles/pay-as-you-go/deal-finder?&dealType=PPAY&phonePrice=0:40#
And remember we do not pay for incoming calls in the UK.