I can't speak for mileage allowances. However many cars are completely unsuitable for people with the spectrum of mobility issues that exist. From needing a higher seat (which tends towards an SUV chassis) to needing enough room for an accessibly wheelchair (which tends towards a larger loading car).
However the single most useful "adaptation" for a disabled motorist is to have an automatic transmission. And you knew that the Motability scheme exists for dealers not customers when you learn that the base model for a Motability contract is the manual version.
The fading of ICE cars is removing that paradox, so it's only right they bring in anew one.
I won't say more - it tends to get me on the naughty step.