I am an American currently living in Britain, but because of the dreadful foreign exchange rate, I'm wondering if it's possible to purchase tickets via a friend or a website in the US and pay with my credit card (also American). Is this possible? Thank you!

Yes. Either have a friend in the States purchase you an electronic ticket using their U.S. credit card (make sure that they purchase it at an airport, ticket office or travel agency so that you won't have to show a credit card upon checkin) or you can purchase an electronic ticket using your American credit card with a billing address in the U.S.