If you've got a debit or credit card, then you can use these to pay for your tickets. If you don't have any cards, don't worry, what you need to do is get someone to buy you a Gift Certificate which you can then use to buy tickets for the NT.

How do Gift Certificates work?

Get someone to call the Box Office on 020 7452 3000 and say they want to buy a Gift Certificate to give to someone to buy tickets at the NT. What they'll get for their money (at least £5) is a special number that you will need to quote to the Box Office. The amount 'on' your Gift Certificate will reduce each time you spend from it, until there's nothing left, at which point you'll need to get another one. (Please note, you'll be able to buy and redeem gift vouchers online very soon.)