This will be contentious, but as someone who always looks after their cars, whether that’s privately owned, company run or leased, I think Motability should only give the full £600 for cars in immaculate condition.
The amount could be reduced for small areas of damage scuffs etc by say £50 for each area of damage.
This way, most users will get a few hundred pounds back and there would be a good incentive to keep the car clean and tidy inside and out.
(Ducks for incoming criticism) ?