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/