Providing the Inster is a 3 month factory order and not a stock car, you cold visit the dealer and ask, if they are willing to place a factory order now and put the Motability application through on your due date. At the same time you can request, should the AP be reduced in Q1, would they be prepared to reduce the amount due by you in line with Q1 prices, if of course AP’s do reduce.
Obviously you need to test drive (extended), to ensure the car is suitable.
I did similar with Ford, at the time we rejoined the scheme and was waiting on paperwork.