No...
The 'advertised' price is an 'invitation to treat', i.e it's an offer of a sales contract, not acceptance of one. So when a customer comes to the till offering £40, and it's the wrong price, the seller can simply refuse the contract offer. Though as soon as they are made aware of a mistake the shop need to change the price straight away or remove it for 24 hours. Internet selling is different but most put in the Ts and Cs that no contract is formed until the goods are dispatched...so that they can correct any obvious errors. Sometimes the honour it for goodwill, but it's only fair do give them the chance to fix it really IMO, anything else is just gives people a chance to rip off businesses who have made a mistake.