Don`t forget that if you cancel on the day of collection the car will have been registered with a number plate and would then be classed as second hand so they could claim the the difference in new and second hand price but it is unlikely.
I think that the only acceptable reason for cancelling on pick-up day is if there is a difference between what you ordered and what is being supplied (wrong spec, missing option etc). In that situation, Motability would fully support a customer walking away, but if it was just because a customer had changed their mind then that’s poor form.
Whilst there is no legal obligation on the customer prior to inputting the PIN on collection day, in my mind if someone is going to cancel an order they should really do so prior to the vehicle being built.