I pay for mine with a Visa debit card.
If you don't have a Visa debit, then talk to your bank. I believe Visa is by far the most useful debit card to have, because I have yet to come across any store who take cards who don't accept Visa.
If you have Maestro (Mastercard) or Visa Electron then you are out of luck, as Apple won't accept this. Halifax for certain do Visa debit cards with their current accounts, amoungst others.