@Keith: There are a couple of different types of plans for Pay As You Go phones. I have not researched this very much, but it seems you either pay a per day fee or you buy minutes that expire (they can be extended by buying more minutes). I think the lower investment is to buy minutes.
Say you buy a card with 90 minutes. The phone is good for 90 minutes or 90 days, whichever comes first. Let's say you use only 60 minutes in 90 days; if you buy another 90 minutes before the 90th day, you then will get to keep your 30 unused minutes, and you will have 120 minutes, and 90 more days of service. But if you don't buy more minutes, the 30 unused minutes vanish, and your phone service is turned off. You can buy another 90 minute phone card and re-activate it at your convenience.
If you come to the US for 3 weeks, and then go back to London, you don't need the phone to stay active while you are in England. You can just let it sit and then reactivate it when you come back. Or, you can lend it to a friend who is coming to the States, and save hiim the $15 or so to buy the phone; all he needs to buy is a card.