Be grateful for credit

I bought a dress to attend a family wedding, paying £350. But when I tried it on again at home, I didn’t like the colour and decided to return it. The shop would only offer a credit note, not a full refund. I am furious. Can I report them to Trading Standards? The shop have done nothing wrong. You are only entitled to a refund if the dress is faulty or not fit for purpose, which is not the case here. The store do not even have to offer credit if you have simply changed your mind.