I think this comes under the same ruling Jacqui that shops can use if they make a mistake when pricing goods up.It is called " Invitation to treat".
This means that whatever price the shop puts on an item it is only inviting you to buy it, not forcing you and can stop the sale at any time.Something like that anyway.Totally wrong in my book, but it is used to cover price c**k ups and costing the shop mega pounds.If it is only a small amount most shops will honour the contract for the sake of customer relations, but as you have found out, some won't.
I could be wrong but this was the ruling when i worked in retail many years ago.
debbiexxx