Exactly. Off the top of my head and as far as I know,in law, this is seen as an "Invitation to Treat" rather than an offer Boots Cash Chemists v Pharmaceutical Society of GB. The shop doesn't have to sell you the product if it doesn't want to, but again, you go into issues of discrimination etc -i.e. what motivation was BEHIND the decision not to sell, or to charge a different price.
There is a difference between an honest mistake and "dodgy" business practices. Things such as the Trade Descriptions Act, or more recently, and more linked to the price, you might want to find the "Consumer protection from Unfair Trading Regulations 2008". These do refer to the price, especially so for vulnerable people etc.
There is also the trading standards issue.