I might be wrong on this, but (I think) you have already accepted the car; by inserting the Motability PIN at the dealership.
If a ‘specific’ feature is an absolute-must, then it should have been checked prior to acceptance – bit late now, though.
As the feature exists on the higher grade model – is that model currently on the Motability list?
Manufacturers reserve the right to change the specification of vehicles at any time.
Can the car that you have be retro-fitted with the pre-heat components?
If so, at what cost – and at who’s expense?