@Adam Don’t forget that by far the biggest cost of a Motability car is the sacrificed benefits of £13,000 over a 3 year lease. Add that £13,000 to those APs and you can indeed buy and run an extremely nice 2nd hand car.
Also, after 3 years you won’t have to hand it back and you can use it as a very sizeable deposit on your next car or, if it’s still serving you fine, pocket the £330 (or whatever it is by then) every 4 weeks!