Not necessarily, no.
There is a 'palpable error' clause in UK law which says that a business does not have to honour prices falsely advertised as a result of gross error or sabotage. For instance if something was meant to cost £199, but due to a printing error was advertised as costing £1.99 - in such a situation a business can claim a palpable error.
If there is no evidence that a communication error was made, they have to honour that price or refuse the sale completely (Invitation to Treat), a failure to do one of the two wil put them in violation of the trade descriptions act - unless fineprint specifies otherwise (which it often does, ie it'll often say something like 'prices are a guide, we reserve the right to determine price at time of purchase').