Unless an item is defective you never have the right to to a refund or exchange on any item. As soon as a sale is completed the purchaser's contract with the vendor is at an end. You could, for example, buy an item of clothing and, 30 seconds later (even before you've got out of the door) realise that you'd picked up the wrong size. If you ask for a refund or an exchange, the retailer has every right to say "Tough luck, mate. Get out of my shop".
There are only two exceptions (relating to non-defective goods):
1. If the retailer displays a sign (or otherwise advertises, e.g. on the back of a receipt) stating that he will exchange or refund unwanted items, that offer forms part of the contract of sale and the retailer must stick to the agreement.
2. If an item is purchased through 'distance selling' (e.g. as an internet sale), there's a 7 day 'cooling off period', enabling the purchaser to return saleable items for a full refund.
Chris