What you do will depend on a number of factors we do not know. For example mainly what sort of car you need, do you need adaptions etc etc
If you simply need a car and your disability does not affect mobility (any size car will be fine). I would say do some research of in stock availability of cars on the scheme and in your advanced payment price bracket, double check the car is in stock and that the dealership are happy to supply on Motability and then if yes cancel the current order, wait for a couple of days (might be quicker) and then get the paperwork done for the in stock car.
I have had two Audi’s in the last 18 months (due to a write off) and in both cases they were stock cars which I picked up 7 days after doing the paperwork.