I wish you would stop referring to designated and allocated drivers, Eddie.
The car cannot be used by anyone, designated or allocated, unless it is for the benefit of the disabled person.
Don't forget a car on the motability scheme has exactly the same tax exemption as a car privately owned by a qualifying disabled person.
It is true that any blue badge holder can park in the disabled home outside cockatiel's home. In fact if there is no blue badge sign on a pole alongside the bay, it is discretionary only and anyone can park there, blue badge or not.