I just saw a bin full of pay-as-you-go phones at Best Buy for 9.99. I'm sure that's unactivated, with no minutes, though. Still not a horrible deal.
As for the option of Google Voice, I'd suggest that only if you have an American cell phone, and if you're considering buying a "burn" phone anyway, the only advantage you'd get with GV would be all the screening features. Not insignificant, but probably not worth the extra trouble, since GV is somewhat less convenient if one doesn't have a smart phone like Android, iPhone or Blackberry, which has either a native or web app specifically for GV, or a readily available computer.
According to what I can tell, when you place and receive calls from and to your GV number, it's just like making a call from the States to Great Britain, i.e. at the international rate. Making calls from your home country to others is, in general, less expensive because you're not using your carrier's network to actually make the call. In your case, where you're traveling away from your home country, you are still making an international call, i.e. from your carrier in England to the U.S., to get to one of GV's numbers, so you aren't saving anything.
Now, getting Google Voice on your American carrier pay-as-you-go phone, in order to make cheap calls back to your friends and family in England might be a good idea.
