UPDATE – I have just spoken to an extremely helpful person at HMRC and managed to get definitive answers to my questions, which they are also putting in writing for me. As an aside, you have to call via the specific HMRC Charities Helpline and there is eventually an option for private individuals enquiring about VAT relief on new cars. Answers as follows;
1) Although my wife is the disabled person, can I buy the car with my money? – answer YES.
2) As my wife cannot drive (due to her disability), can I be the registered keeper? – answer NO. My wife has to be the registered keeper and I would be the nominated driver.
3) As I am the only driver and we therefore only need one car, is it acceptable for me to use the car on my own, for purposes that are of no direct benefit to my wife? – answer YES.
This last one was a pleasant surprise, as the guidance rather implies otherwise, but she said that the principle is that the car is permanently adapted so that on the occasions that my wife needs to use it she can but, as the nominated driver, I can use the car for my own purposes. I pressed her about scenarios such as visiting family and friends, day trips etc all of which could be on my own and without my wife, and she confirmed that all such journeys are absolutely fine. This now explains why I got exactly the same answer when I asked Motability all these questions when we retired (and went down to one car). The answers that Motability gave me are therefore in line with HMRC guidance. So, in summary, I could use the car for whatever purpose that I like and need not worry.
She stressed that you must make sure that the dealer applies the VAT exemption at source as it cannot be refunded if applied in error. She also said that dealers are not obliged to partake in the VAT exemption scheme and some Mercedes and BMW dealers are refusing to do so, trying to push people into Motability cars instead (she didn’t say, but I wonder if that’s to try to get their EV targets up?). As I would be looking at a car that was way above what is available on Motability hopefully I wouldn’t have this issue, but I’d need to check first. Thankfully I have a great contact at our local BMW dealers.