Not illegal at all. Even if the wrong price is displayed the retailer has no legal responsibility to honour the price, as all displayed prices are an invitation to barter.
Trading Standards can make recommendations to retailers regarding trading practices, but I believe they have little power to enforce anything, the retailer may need to show due diligence, however there are many loopholes.
All that being said in the age of social media, the customer has truly become king, and most large businesses will honour wrong prices etc as an act of goodwill in order to give “good service”.