Why can't I pay for apps with my App Store voucher?

I recently bought a £20 App Store voucher for my iPhone (running iOS 6) and, even though I entered the code and it was accepted by Apple, my phone still fails to recognise the fact that I have £20 as my balance. Instead, it asks me to input credit card information (which I didn't do, presuming that it would charge the App cost to my card as opposed to the voucher). I wondered if there was any setting to configure which way you pay for Apps etc.

You can buy 20 pound cards it's 15 25 50 and 250

Because I'm Batman!