Taken from...
http://www.solihull.gov.uk/tradingstandards/pricingofgoodsandservices.htm
What the Law Says
Normally, when goods are offered by a retailer there must be a written indication of the selling price
If goods are exposed for sale, the price can be on the goods themselves or on a ticket price list close to them. If the goods are not on view, the price must be on a price list or in an advertisement or catalogue
Each price must be unambiguous, easily identifiable with the goods, and clearly legible
Any price given for goods or services must not be misleading
All retail prices must include VAT. If there are any non-optional extras which must be purchased with the goods, their price must either be included in the main price or displayed just as prominently together with an explanatory statement
Price comparisons should be factual. Do not use descriptions such as 'worth' or 'valued at'. The abbreviation 'RRP' and 'man, rec. price' are permissible. All other abbreviations should be avoided.
All price companies must be clearly explained and must not mislead in any way. You should compare like with like and where a reduced price is claimed then the product should have been offered for sale at the Higher price for at least 28 days in the previous 6 months in the same outlet. If your comparison does not meet these criteria then you should provide an explanation which is not ambiguous, easily identifiable and clearly legible to the consumer.
But, taken from....
http://www.telford.gov.uk/Advice+benefits/Trading+standards/Trading+Standards+FAQs.htm
Q. I have seen a CD player on sale in a shop that I want to buy. However, when I tried to buy it the manager told me it was the wrong price. Can I make the shop sell it to me at the displayed price?
A. No. You cannot make a trader sell anything to you if they don't want to. In civil law when goods are displayed in a shop, a catalogue or an advertisement, this is called an 'invitation to treat'. This means that it is an invitation for the consumer to make an offer to the shop to buy the goods, and it is up to the shop whether or not they want to accept that offer.
However the shop might have committed a criminal offence. This is because the Consumer Protection Act 1987 says that traders must not display misleading prices. You could report this to Trading Standards who may investigate the matter, however if this was a 'one-off' genuine mistake they would be unlikely to take any action.