Do you live in the UK and are you using an account from a card company in the UK with a UK billing address?
The iTunes Store in a country is intended only for use by that country's residents while they are in the country. To use the iTunes Store in a country you need a credit card (or other card type if acceptable in a country) issued in that country, billed to an address in that country, and also be physically present in that country when using the store. You are also restricted to waiting 90 days between country changes.
E.g., "The App and Book Services are available to you only in the United States, its territories, and possessions. You agree not to use or attempt to use the App and Book Services from outside these locations. Apple may use technologies to verify your compliance." - http://www.apple.com/legal/itunes/us/terms.html#APPS
Maybe something here:
Does your card belong to one of the ones listed?
Hold on, that's for the Apple Store, sorry. Maybe you should contacct iTunes support. iTunes Customer Service Contact - http://www.apple.com/support/itunes/contact.html