Some banks and credit card companies offer single-use or virtual credit card numbers. Citibank is one, I believe. I don't know if they're available in the UK or not though.

For example:
http://www.citibank.com/us/cards/cardserv/advice/van.htm

If someone gets that number from a website database, it won't do them much good. Other alternatives are pre-paid credit cards, if you want something physical.