When you buy something from a shop you are entering into a legally binding contract. Therefore they don’t have to give you a refund simply because you have changed your mind. Only if one of your statutory rights is breached (i.e. that the item is damaged, of poor quality or not fit for purpose) do they have to give you your money back.
You have no right in law to cancel the agreement and if you do you could lose the deposit. Sometimes shops will be generous and allow you to transfer the deposit to another product, which in fairness has been agreed regarding the guitar in your situation. However, this is at the discretion of the shop and you do not have any right to demand that they give you a refund or even credit you with your deposit if you cancel an order.