Why can't I use my £15 credit

What happens when you try to buy something, does the 'buy' button not work, do you get any error messages ... ?

If you are getting a message to contact iTunes support then you can do so via this page and ask them why the message is appearing (these are effectively user-to-user forums, we won't know why) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page

If it's a different problem ... ?