Further to CC's excellent (and completely correct) answer:
When you buy something in a shop, you're entering into a contract under which you agree to pay a certain amount in exchange for certain goods. As soon as you've handed over the money, and received the goods, the contract has been completed and there are no further obligations upon either side.
So, for example, you might buy an item of clothing for �20. Once you've bought the item, the retailer can't then stop you and say "Sorry, I've decided that the price is �30. Hand over another tenner". Equally, you can't make any demands upon the retailer. Even if you realise, just one second after completing the purchase, that you've bought the wrong size, you have no right to demand a refund or exchange. If you attempt to do so, the retailer can legally say "Tough. Get out of my shop". The retailer is not responsible for your mistakes. (Why should he be?)
There are, of course, separate legal provisions relating to faulty goods (or where, for example, the retailer had attached a ticket indicating an incorrect size) but the general rule is that once you've bought an item, there are no further obligations upon the seller.
Chris