Walk into shop. Buy one.

The best deals are in Asian-run this-&-that shops which also offer mobile unlocking services and international calling cards. Locally the going rate at those places is 3.50 pounds for a prepaid card with any of the major services (Orange, O2, T-Mobile, Vodafone, Virgin). From a high street shop run by one of those firms, it will cost you several times as much and you'll have to put up with a creepy spiel aimed at getting you to sign up for a contract Faust would have recoiled at.