You're probably going to be best off using cingular. Roaming on prepaid SIMs can be difficult. It used to be impossible, though today it is more common, however it is usually expensive (£2 per minute) and has limited availability. You may also have to use a call back feature where you enter a code and the number you want to call and then the phone calls you back to complete the call.

You may want to check out the websites of various UK mobile phone providers to see what's available and if it makes any sense. I know that in Germany D2-Vodafone offers prepaid roaming, so I would suggest checking with vodafone first. Other UK providers include orange and BT.

Perhaps you should consider switching to voicestream, which has much cheaper rates. Or you could just sign up for voicestream service and use their roaming.

There is also one other potential problem -- your phone may have a subsidy lock (aka SIM lock). This prevents you from using a SIM from someone other than the company that sold you the phone (because they subsidize the cost of the phone). Cingular would have to release the lock for you to use another SIM.