Hi James.
I don’t like to post this but please see the below link.
https://www.motability.co.uk/contact/faqs/can-i-change-my-mind-after-i-have-made-a-car-application
However, if you want to cancel due to the dealer misleading you in any way then you’re protected by the consumer rights act. i.e. If the dealer did not bring to your attention a key change or led you to believe the vehicle has something it doesn’t.
https://www.legislation.gov.uk/uksi/2008/1277/contents/made
Good luck either way and keep up posted ?
? I will be remembered for nothing but had great fun doing it ?