I'd look into Orange --- from what I understand they don't care for non-residents registering ( though they used to, and may still do, allow an hotel as a valid address ) --- but here is their Camel Pay-As-You-Go sim, if you hit show details you can see the rates for each country which average 15p a minute ( but 6p for the USA ).
http://shop.orange.co.uk/mobile-phones/plans/paygSimPlanList.jsp?selectedTariffName=Camel
You could prolly get someone in England to post you one, or buy one off Ebay.co.uk.
If there was difficulty registering to a non-British credit card ( and that's not a given ), nearly every other damn shop in Britain sells top-ups for all the sim providers. Alternatively, starter packs of sims are sold in stores, but those would not have cheap roaming.
Here are a couple of quite recent links discussing this problem:
http://www.ricksteves.com/graffiti/graffiti134.html
http://news.cnet.com/8301-1035_3-10251727-94.html
The latter affirms: 'Buying a SIM card when you get to London from a local operator, such as Orange, can offer even better deals. Orange offers a variety of service options for prepaid customers. In general, domestic calls range from 30 cents to 40 cents a minute, depending on the exchange rate. (All calls are billed in local currency.) And texts are about 20 cents to send and receive. With a special international plan, customers can also make international calls for as low as 10 cents a minute. Orange also offers free text messaging for customers when they "top up" or add money to their phones. One plan offers 300 free text messages with a 10-pound top up. And you can get 600 free text messages with a 20-pound top up and a 30-pound top up gets you unlimited text messages until the card expires.'