Your purchase is protected by the 1979 Sale of goods act, as amended and improved by the 1994 act, regardless of the goods being in a sale or marked shop-soiled, seconds or imperfect they must be of satisfactory quality, if a fault is pointed out to you and you decide to proceed you cannot later return the item on the basis of the fault you earlier accepted.
What you are not permitted to do under the act is to change your mind, if the goods are of satisfactory quality and as described you are unable to blame the retailer because you made a mistake or changed your mind, it is expected that you were given the opportunity to inspect the items before deciding to purchase, if the items were the wrong size due to being incorrectly labelled this would be a breach of description under the sale of goods act. They do not need to provide a credit note just accept it.
I nearly was divorced for giving similar advice to my wife.