You need to create an account in that country. When you create the account, it asks you for a credit card form that country. To get around this, you can purchase a gift card form that country and when you log into the account, click on redeem and enter the gift card number. Then you can purchase stuff from that countries iTunes. I'm not sure what happens if you then try to use your credit card after you have used up the gift card. You may need to keep purchasing gift cards form that country. I have bought US gift cards form eBay but I can't recall seeing any UK gift cards.
Cheers.