As above. You call Motability, not the dealer. You shouldn’t have any difficulty getting Motability to agree to an early termination. Sacrificing £4,000 benefits for just 500 miles is clearly not viable and they will understand that. Good luck.
Just to add to this good advice
You can return to ANY
I suggest nearest Motorbility dealer no matter the make
When decided where that is to be inform motorbility who will give said dealer your date of return
Maybe take a few pics prior to return
For peace of mind in case of any future quibbles