There are a few qualifying factors including adaptations and options that you’ve had to put on at your own expense. I checked with Motability and I am able to extend the lease having spent a lot of money to make my vehicle suitable for me.
As regards your second point about reaching the end of a five year lease, I will have two more years in which to identify a suitable replacement. The chances are that this vehicle will become available at some point and I can cancel my extension at any time.
That is what I call a good deal.