I cant see what the problem is really.
Motability lease you a car for 3 years, that is the contract you sign.
They are under no obligation to offer you the car once your contract ends.
Given that they did indeed at one time offer the vehicle to users after the contract ended is neither here nor there.
It is quite a large company and will have an extensive legal department who will make sure they work within the confines of the legal contract.
Unless you are a qualified legal whizzkid, I cannot see a resolution to your concerns.
Indeed, draughting a nice letter thanking Motability for the scheme and suggesting re-instating the option to buy at contract end would more likely achieve a better outcome than complaining to the financial ombudsman or whatever action you took.