Tesco supermarkets sell their own SIM cards. I think that they actual use the O2 network.

They charge about 5 UK Pounds for it and if you put on a minimum of 15 pounds you get calls at 10pence/minute. Their rates are cheap to the US too.

If this is of interest to you, you can google Tesco, or try www.tesco.com or www.tesco.co.uk

I travel to the UK with an unlocked GSM phone, and just switch SIM cards depending if I'm in the US or UK.