I think doing this is why dealerships ask for a deposit because if everyone done this they would lose to much money as they would always be ordering cars but with no guarantee of the person buying it.
im not sure how much Motability pay for a vehicle but once you order a car they might either leave it in the compound n sell it new and lose less money on it or pre reg it and sell at a drastic discount. Either way I would feel bad for the dealership but as it’s your money you’re entitled to do as you please so if you change your mind you have the right to do so