Although a risk if you definitely want to buy your car at the end of the lease, this is definitely worth considering. The prices that Motability quote their customers who want to buy their car are clearly designed to deter all but the foolhardy.
I ask each time out of interest and for a car sold as seen with no warranty they have always asked silly money. For our last car, I found it for sale a few months later at a dealer in Norwich and they were asking £1k less than I was quoted, even with a warranty and so I suspect that they paid at least £2k less than that at auction.
I have just asked for a price for our current car as the lease is almost out and, yet again, they are asking £3k more than the car would fetch at an auction. Given that a 5% discount should be fairly easy to negotiate on a second hand car, I could even buy a similar car from a main dealer for less and that would have a warranty!