My understanding of the law is that an item that has the wrong price displayed has to be sold at that price.
I don't think thats the case.
My understanding is that a retailer can decide not to sell you the item when you get to the till before you pay, however if you've paid for the item then the displayed price must be the price charged ie they must realise its wrongly priced before you pay not afterwards. If they display one price and charge another they have committed an offence, however they can legally decide not to sell you the item provided its done before your offer is accepted (acceptance of payment is usually considered to be the point at which the contract is formed, and that seems to be the debatable issue with this B+Q offer.)
One of the most rampant offenders of wrongly displayed prices in the UK is Tescos. They frequently charge you more at the till than is displayed in store. The problems are endemic within their systems and have been for years. They've been fined heavily in other countries for this but the UK seems unable to do anything despite it being an offence.
