Generally, UK retailers are free to set their own prices and price controls do not exist. In a competitive retail market, if a trader sets his prices too high, consumers will purchase the products they want from a more reasonably priced retailer.
The law requires traders to show the full selling price, including VAT, of all products they offer for sale to consumers. They must also show a unit price for products sold from bulk such as fresh fruit and vegetables (which is a price per kilogram, litre etc.) and certain products marked with an indication of quantity or pre-packed in a constant quantity. For example, a bag of ten oranges costing £5.00 might display the unit price as 50p per orange. These price indications must be unambiguous, easily identifiable and clearly legible and be inclusive of VAT and any other taxes.
It is a criminal offence for traders to give a misleading price indication about goods or services. That applies in whatever way the price indication is given, whether written in a notice or leaflet or given verbally. For example, you may see a shelf edge price ticket showing £5 for the item, and then you are charged £5.10 at the checkout. Another misleading price indication may be a price indicated on a leaflet, but you are charged at a higher price in the store.
You can't insist on buying a product for the displayed price if a trader prices a product with the wrong price in error (for example, "28 inch widescreen television for £39.90" instead of £399), unless the transaction has already been completed. The trader must nevertheless take immediate steps to rectify the mistake.
If you order goods for an agreed price, the seller cannot increase the price when the goods are delivered, unless you have agreed in advance that the price may change. If there is a significant increase in the price, you may be able to argue that this is an unfair term and you are not bound by it. This will not apply if you were given the right to cancel should the final price be too high.