There is no law that states any retailer should take an item back due to it being unsuitable. A contract is formed between seller and buyer upon exchange of payment. Only faulty items are regulated by law.
Go along with most of that BBK, but it's a bit of a sweeping statement. Formerly "distance selling", now Consumer Contracts Regulations, allow you to cancel many items within seven working days from the day after you recieve your goods.
Sorry, I forgot about distance selling regulations in my answer. I tend to deal with non distance selling retail.