You should be able to use any MINI dealer then. Like Sytner or Soper etc.
Personally... Not heard of Marshall in the bmw/mini scene before...
Shows how much I know, I always thought they were an official dealer, they do a very good job of looking official then, sell brand new cars too. I just typed their address in at mini.co.uk and they don't come up so it looks like I screwed up there too. Not that I had much choice mind, we bought this car right as lockdown hit so choices were rather limited.