Kayla,
Have you heard of Mobal.com? They offer a GSM World phone good for use in 150 countries for $49 and it includes $20 in calls. They also offer a $99 phone that woeks in 170 countries including the USA and includes $99 in calls.
They give you a British phone number so your calls in England would be very cheap. The phone and number never expire. You can toss it in a drawer after your trip and still have it to use on your next trip. (They do recommend making one call every few months though, just to keep it current.) The charges are made to your credit card, so you don't have to buy minutes.
I used the $99 version during my trip to Paris and Switzerland in August, 2008 and it worked flawlessly. Never found a place, even in the Swiss Alps, where I had no signal. And my sister in the US was amazed at the call clarity. The per minute rate is NOT a bargain. Paid about $1.50 per minute to the US, but if you will use it only sporadically, it's not so bad given the cost of the phone. The $99 phone gives you more than an hour of talk time. The $49 phone gives you 13 minutes. But since it's charged to your credit card, you don't have to worry about buying minutes you may never use.
At any rate, you might want to check it out and see if it would be an good option for you. Mobal.com