If motability did help which I think they won’t, but if they did they will just take your mobility component money and let you hire a car. So why not just use your mobility money and hire yourself a car in the meantime time.
You could cancel the order if you find a stock car that suits which you could get in between 3 days to two weeks.
What’s the plan with old car. Fix and sell, trade in or scrap? If the first two then you need it fixed anyway. Get it done asap so you have a car until your original order is ready.