The current Terms & Conditions don’t mention anything about return of your AP under any circumstances if you cancel the agreement. Here is the latest agreement for cars since 19th April 2017:
https://www.motability.co.uk/car_wav_terms_and_conditions_07-17.pdf
Found here:
https://www.motability.co.uk/information-for-customers/terms-and-conditions/
Pertinent section is #16.