It rather depends on the level of disability I think, depending how mobile one is.
Many yards will be able to cope with visually disabled riders/handlers, for wheelchair access requirement may be slightly more specialised, good level access, ramps etc.
I would suggest that you contact the RDA, they will have lists of Riding Schools/Equestrian Centres etc local to you that have facilities and are able to accomodate specific needs.
Also, I would just ring around and specify exactly what is needed.
Good Luck, I am sure that there are many yards that can help.
It might be too far but you could check out the Diamond Riding Centre in Woodmansterne (near Banstead), Surrey. Not sure if they do livery but they are a big RDA centre. And Im sure they would know if there is anywhere that does, and would be suitable.
This is probably so out of date it isn't even relevant any more!!
However, I used to live in Godalming, and rode in Elstead at Truxford. Back in about 1994/1995 my Dad helped construct a mounting block, with ramp etc for the RDA use.
I have no idea if the place is even still there, and if the ramp is still there if it is. However the yard was flat, and brick paved at the time - Dad helped lay it, it took them several days!
How about looking at places that have RDA groups there (as has been said).
Langshot RC at Chobham are livery & a RS - they also have RDA group there so are set up for disabled access in car parks etc & schools. Indoor & outdoor schools & access onto hacking without roadwork straight onto Chobham Common.
Also, Shey Copse RS ( off Old Woking Road on outskirts of Woking in Pyrford) have a specific RDA group as well as usual lessons & liveries - tho I know their hacking is only a couple of bridleways.
Would be worth checking with the BHS ref approved livery yards & then visit a few locally and ask questions.
I know its not a question most will ask - but a YM/YO will: how much assistance will you need?
Are you mobile or in a chair or using a frame? Do you need 1 to 1 assistance whilst out of the car at all times, or assistance till mounted & riding away?
Can help you check round locally if you wish - drop me a PM if you need help in finding somewhere