You have to apply for a blue badge and you need to think carefully about what you write in the application form. I had one for mum butbshenhas a wheelchair. I was less confident about getting one for Bill but I got it.
I got a free radar key for disabled access toilets from the council. I used Bill's attendance allowance letter as proof of disability. I think this is also what I used when I bout the disabled person's rail card.
Assistance at airports requires no proof of disability.
The disability is mental incapacity as far as I'm concerned.