Actually in this case the law is entirely on your side.
Your contract doesn't officially start with a retailer until they dispatch your goods. You can cancel your order at anytime before they dispatch it. As they have not yet dispatched the goods on your order, you are well within your rights to cancel your order and request a full refund. It is then up to them to arrange for the incorrect goods to be collected.
I've had numerous experiences from online department stores sending incorrect goods. I've requested either my goods to be dispatched immediately, or a full refund. I also inform the retailer that in either case, if they require the wrong goods that they have sent me, it's down to them to arrange a collection.
Last year i must have sold £100+ worth of incorrectly sent items on ebay as the retailer hasn't even bothered to arrange collection.