based on a quick reading , youre talking about the difference between distance selling and shop purchase.
Sales in person (the latter) are based upon an ability to examine the goods, ask questions and then accept or reject the offer sell based on that opportunity. You can always walk away from the shop. Some shops will allow you to return goods, but thats just goodwill.
The former allows cancellation as you dont get the chance to inspect [b]before [/b]you pay. I [b]believe [/b]that this also applies to purchases based on a phone description as well as electronic purchases from a business as they are similar. You didnt buy a specific item in advance (they had two, and you didnt pay up front) and you did get a chance to try it and to inspect. You made your special requirements known and they seem to have gone [i]some [/i]way towards accommodating you.
I dont think you have a legal case, but they should probably help you out based on good will.