The Distance selling act (I believe) is in place to protect you from mis-selling or "dodgy Dealers" as you would not have been able to inspect the goods before buying them. If you purchased an item in store it would reasonable to assume you had chance to look at the product before your paid for it, therefore establishing if it was suitable or not. However, (at store discretion) you may find a manager would be prepared to exchange the goods if:
They were returned within a short time period and also in pristine re-saleable condition. If the product is old and shows sign of wear it is unlikely a return would be offered. This is simply because no one would want to buy a used mouse and keyboard in a crumpled box. Hope this helps and you find a more suitable set soon!