Just as a further point of interest as I don’t know what make of car that the OP has. Although premium manufacturers are no longer able to take new Motability vehicle orders, it may still be possible to drop off cars that are being returned early to them.
We are doing an early return on our Motability Hyundai, and the BMW dealer that I am buying a new private car from has said that we can leave the Motability car with them on the day that we take delivery of the new BMW. Due to them still servicing Motability BMW cars for the next few years, they still have access to the system, so can process an early surrender as well.
Motability confirmed to me that we can return the car to any dealership that has access to their system. It doesn’t have to be the supplying dealer, or even the same brand.