If the car is written-off, then my understanding is that the car would be replaced under the insurance policy with a ‘current like-for-like’ model. From my policy, under ‘Market Value’, “The cost of replacing the Vehicle with a vehicle of the same make, model specification, mileage and age, and which is in the same condition as the Vehicle was immediately before the loss or damage which is being claimed for.”
If Motability car is written off your current agreement ends and you have to order a new car and start again. You get a pro rata refund of any advance payment BUT do not get any money back for extras you may have paid for whether factory fitted or fitted after delivery.