I think Motability would be more likely to offer you an extension of lease upto 5 years on your current car, than give you a grant, particularly if your current vehicle is still suitable disability wise. Fuel cost isn’t a consideration with grants as Phil has pointed out above, thus any grant would probably be for a standard diesel as opposed to the more expensive PHEV varient , so you may be back at square one with fuel cost. Another way maybe to see if you maybe able to get a grant from a charity other than Motability. For example if you have served in HM Forces, there are a multitude of ex-service charities who maybe able to help. Or is there a charity specifically connected to your disability? Or search here: https://grants-search.turn2us.org.uk/
I always look at the 3 year lifetime costs when shortlisting a car, here is my example, the AP and the fuel costs over the 3 year lease period, my current car a Ford Kuga 2.0 Diesel Powershift original AP £1295 plus fuel costs estimated @ 7,000 miles a year (including towing a caravan 1500 miles a year) £3450 = £4745.
Mitsubishi Outlander PHEV AP £1699 fuel cost @ 7,000 including towing £1905 = £3600.
That a saving over the 3 year lease £1145
Unfortunately I have suffered a brain injury and occasionally I get confused and often say the wrong thing.