This is from the Motability website, for Eddie's benefit:
We’ll ask you to make sure:
The car is used by, or for the benefit of, the disabled person. This does not mean that the disabled person needs to be in the car for every journey. In practice, this means other named drivers in the household can use the car for shopping and other routine activities, as long as the disabled customer will benefit
http://www.motability.co.uk/about-the-scheme/how-your-car-can-be-used/
So you see, the designated driver cannot use the motability car for his own use.