Hi Kim
My thoughts are roof boxes fall into the same category as dash-cams in that there supposed to be fitted by the dealer who of course add hundreds of £’s to the cost.
Not ever had a roof box fitted to my Motability car (just too hard getting the scooter up and onto the roof? ) but I do fit my own dash-cams that is against the rules but its one of those thing like, who’s to know and the first thing Motability insurers ask after an accident is “do you have any dash-cam footage”.
Its simple to take cameras and roof boxes back off the car and none the wiser although you have to remember the insurance side of things should your box fly off on a motorway or whatever so bear that in mind.
Can’t say what Motability will say if asked but others may help here.