I agree, most of the way, but a genuinely, seriously disabled person. , needs a decent, reliable car, that is properly maintained. The last thing they need is some unreliable banger. The Motability scheme, pulls everything together, gets big discounts out of suppliers, as well as avoiding the disabled paying VAT .
I'm now disabled, like you, have to self fund my own disability, and I cannot even get away with the VAT, on most things - except by buying from disabled type supply companies, paying well over the odds in the first place. Everything for the disabled, costs two/three times as much as it is worth, if it carries the 'disabled' tag.
The mobility scooter I bought second-hand, unused, was two years old, fitted with new batteries. It cost me around £925, new it would have cost £1600, because the company is a Polish company, selling direct to the public, bypassing the UK disabled dealers markup. The scooter is considerably better, than most that I could buy in the UK dealers, as well built, much better specced etc..
If the Polish company, actually allowed their machines to be sold by UK companies, then I would see those machines sold in the shops for maybe £5000, VAT free. Even paying these silly prices, a lightly used, not particularly old one, can be had for just £350, because used ones, realise their true value.