You can use any Motability dealer to get your vehicle serviced.
Whilst generally it will be a franchised dealer of the same manufactuer, it doesn’t necessarily have to be. This is particularly so if it is a long way to get to the nearest dealership. I believe someone on here had their Ford serviced at a Renault dealership (or it may have been the other way around) up in Scotland as the closest franchised dealership was many miles away.
If you are wanting to get your car serviced away from the supplying dealer, let Motability know so they can assign your vehicle to the relevant servicing dealership.
Mike