Not a leg to stand on i am afraid, if thats thir policy, then thats the end of it, from Trading standards website:
Q. I bought a jacket for my daughter the other day, but when she tried it on she didn't like it. I thought I was entitled to have my money back, but the shop has said no, and will only exchange it or give me a credit note. Are they allowed to do this?
A. Yes. The law only requires a shop to take goods back and refund if they are in breach of contract, in other words if the goods are faulty or incorrectly described. If you have made the wrong choice, then they don't have to do anything, not even offer a credit note or exchange, so what they have offered is more than they have to.
Of course, many shops have very generous policies and allow you to bring back unwanted goods, but crucially, they don't have to. If in doubt, check with the company before you buy, and if necessary get them to write on your receipt if they agree to offer a refund.