Is this any use?
http://www.bbc.co.uk/consumer/your_rights/goods.shtml
retailer doesn't have to sell an item at the marked price
Generally speaking, in contract law terms, a retailer doesn't have to sell an item at the marked price. For there to be a binding contract there has to be an offer which has been accepted. In the context of sale of goods, the offer is made by the customer when they present the goods at the payment point (if the customer offers to buy the goods at the marked price).
The till operator is then free to accept, or reject, that offer. This means that if a till operator spots that goods have been mispriced, they can reject a customer's offer to buy at the incorrect price, and offer the customer the chance to buy at the correct price.
The customer may decline or accept that offer as he or she chooses. Inevitably, retailers will make the odd mistake over pricing, but if you become aware that a certain retailer makes a regular habit of doing it, we suggest you report the retailer to the trading standards department of your local authority. In certain circumstances, misleading pricing will amount to a criminal offence, and trading standards departments have the authority to investigate, and, if appropriate, prosecute.