Although not an exact statement of law, the Motability website does state in relation to use of a Motability vehicle:
”Using it for business reasons we haven’t agreed with you, like as a taxi or for deliveries” (my bold) – bullet point five below:
https://www.motability.co.uk/whats-available/cars/package/use/
The ‘like as a taxi or for deliveries’ are clearly exceptions to business usage ‘agreed with both Motability/RSA and the customer’.