Motability tell us it’s fine to cancel right up until you put in your PIN number. So I wouldn’t think twice if a better more suitable car came on the scheme and I was prepared to wait again for delivery.
If you cancel after you’ve collected it they must have worked out is costs them in clerical time as they say there’s a fee of £250.
Dealers will easily sell on any car cancelled and delivered to them and likely make more profit then from motability.