Hi @speedysloth....love that username!
I would think that it goes without saying that without permission from Motability you would be in serious trouble, not the least of which would be that you would have no insurance. Add that to driving without the owner's consent and it's a high-risk game to play. Please save yourself the hassle by sorting this out ASAP and possibly giving yourself time to formulate a Plan B.
It could probably be sorted in 5 minutes over the telephone.