You can fit different makes of tyre on the same or different axles, providing they’re all the same size and type of construction and have the same tread pattern. Construction isn’t an issue anymore due to the rarity of radial tyres.
However it’s generally considered, that both tyres on the same axle should be replaced in pairs, rather than mixing brands but, I don’t believe it is illegal.
That said I wouldn’t allow it on my car, mainly because I did my apprenticeship at the Michelin and the last time KF tried this, I declined and got their mobile unit to come out after requesting the correct tyre.
If you are at all unsure/not happy speak with Motability and tell them if they must fit a different brand, you want them to be fitted in pairs. You can read aloud, what Michelin or any the manufacturer says.