Legally speaking this is a fairly complicated matter with questions over the car specification vs the actual car and it is also complicated further as this is not just an agreement between you and the dealer but between you, motability and the dealership. From a legal perspective it will depend on the terms and conditions of sale, if you were made aware of any changes etc.
Practically speaking however, Motability sometimes take a pragmatic view, and sometimes not. I was supplied a BMW X1 which should have had auto parking fitted, but did not, or appeared not to. When I spoke with motability they made it clear that having accepted the car, it was my responsibility and there was no prospect whatsoever of an early termination. As it turned out the car did have it fitted and all was OK. Confusion down to very poor customer service and lack of product knowledge. In your case the circumstances are different and the outcome could be different. I do not believe there is a cooling off period, others may confirm but this has been a point of discussion man times in the past. Good luck.