From our experience, the key is to get a written confirmation from the Motability sales agent of any cancellation costs if you were to decide on a cancellation before delivery.
In our case, we had ordered a Vauxhall Astra 1.5 diesel Ultimate last July (£1,599 advanced payment at that time) and by end of January this year (6 months) we still had not been given a build date. Had we been, I wouldn’t have cancelled. As it was, we were honestly fed up of being left in the dark as unless we approached the dealer, we heard absolutely sweet fanny adams.
Consequently, we cancelled the Astra once we had written clarification of zero cancellation fees, and went out the next day, (once we checked the Astra order had cleared from our Motability account) and ordered a new Renault Zoe E Techno R135 Boost Charge for nil advance payment, thus saving £1,599. We did this two weeks ago and should have the car within the next ten days. What a differences a day makes.