My mother gets motability and has an ev so where does this fit in between the standard model where middle classes buy ev vehicles and poor people buy used ice vehicles
In the case of Stellantis I believe this is driven by "compliance" requirements - the need to sell a certain percentage of EVs to avoid being fined by the EU. So they're not especially great EVs, but once they go to the lease firm they're counted as "sold". Someone may take a loss on the depreciation later, but not the driver.
A special consideration for Motability is that PIP recipients aren't allowed to save up for anything, so whatever vehicle they have has to be on finance.
That's incorrect.
Some, not all, PIP recipients are also in receipt of means-tested benefits, such as Universal Credit for example, which has a £6k capital disregard, and a reduction of benefits between £6k and £16k, and no entitlement over £16k. (Also I'm sure there are PIP recipients who work and receive no other benefits)
There surely are PIP recipients who would end up with savings of over £6k if they did not spend their PIP on anything else, and Motability is a legitimate way to spend it in the eyes of the DWP.