Hardly something worth warning about as this is the standard for most ebook stores around the world.
However, you have 2 options that i can think of if you have a Paypal account.
1. Someone with a WH Smiths account can buy a gift certificate and you pay them back via Paypal.
2. Buy your books from BooksOnBoard as they accept Paypal as a payment method.
Or use the rather hopeless Waterstones store