Just checked Trading Standards and it says:
Under law, a retailer is entitled to decide the price he wants to charge for his goods.
The price on display is simply what the law calls an ‘invitation to treat’. In the same way you dont have to buy goods from anyone, a retailer is under no obligation to sell you anything.
You cannot insist that a shop sells you anything at a marked price, whether or not they have made a mistake.
The law does not allow prices to be fixed and, contrary to common belief, goods are not subject to price controls.