Here is what the BBC has to say about internet pricing errors:
http://news.bbc.co.uk/1/hi/technology/2872429.stm
The relevant paragraph is probably this, since UK law is based on case law (ie previous decisions on similar issues are used as the basis for future claims): "
case law makes it clear that if a buyer understands that a price is a mistake and tries to take advantage of it then they have no right to insist on the offered price being upheld."