It’s very much like any grant application, if you look at it from their point of view, you need to very clearly why it is in their interest to give you more money than minimally necessary.
I would start off by we need a car in X category, so we went to every dealership that offered one on scheme and test drove them. The Karoq was the only one that offered the seats that meant my wife could sit in without suffering with back issues. The one we test drove was a higher spec than base and, in our opinion, would be the spec we would require for seat comfort, adjustibility and (pick an exclusive, convenience based feature that requires this spec, like auto tail gate…) for maximum benefit and to make the car fully suitable to both her current needs and likely degeneration of her condition over the next 3 years.
As said, being firm guides them to your conclusion and being polite with it increases the likelihood of them agreeing.
Good luck.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark