They have to follow UK Law for any sale made within the UK and this is certainly against British Consumer law. We do have much better protection over here than the US. There is a clear breach of the Consumer Protection code of 2005 with regard to the previous price.
1.2.5. For any food and drink you need not give a positive explanation if the previous price in a comparison has not applied for 28 consecutive days, provided it was the last price at which the goods were on sale in the previous 6 months and applied in the same outlet where the reduced price is now being offered. This also applies to non-food items, if they have a shelf-life of less than 6 weeks.
Also
2.1Indicating two different prices
2.1.1. The Consumer Protection Act 1987 makes it an offence to indicate a price for goods or services which is lower than the one that actually applies. You should not therefore show one price in an advertisement, website, window display, shelf marking or on the item itself, and then charge a higher price at the point of sale or checkout. In addition, specific regulations apply to particular types of sales and ways of selling—eg retail sales (including the Internet), sales of food and drink which involve service, distance contracts, resale of tickets, package travel, etc. Your local Trading Standards Services or Home Authority will be pleased to advise you on the current regulations that are relevant to your business and of any good practice guidance which is also relevant.