PIP for returning British Citizen

I have late stage MS (and, for my troubles, advanced Neuro Myelitis Optica or NMO) and live in SE Queensland in Australia.
I am thinking of returning to the UK (I was born in North Yorkshire) and can probably afford to buy a flat in the North East (based on property searches!)
I understand that I would have to complete an application for a PIP to get a benefit and that, as my mobility is severely restricted, I may be able to get a vehicle through Motability (I need an automatic now, as depressing pedals with my left foot to change gear is extremely tiring/not happening).
I saw the warning re wait time for PIP approval, but that this was an average wait time and severe cases would be viewed more quickly?