The dealer paperwork for our new car had a clause stating that the price may be subject to change if the car wasn’t delivered before 31st October. Not sure how that would have worked on Motability though, would that have been our problem or Motability’s? I would hope and expect that Motability would have made up any shortfall, I’m sure they’ve allowed a budget for transitional variations.
Dont forget that Motability have guaranteed Prices/Advance Payments for cars ordered on or before 31st October 2019:
Due to uncertainty around the exact arrangements for the UK’s expected departure from the European Union, these prices may be subject to change after 31 October 2019. The prices quoted are guaranteed for cars ordered on or before 31 October 2019 and will apply even if this changes while you are waiting to take delivery of your car (the exception being where the car description is changed subsequent to the original order.)
I expect they will move the guarantee date along at some point, possibly on a rolling monthly basis, pending any formal Brexit deal happening/not happening.