Technically, but not really true any more, someone requires standing to bring a case to the high courts. Standing means you have been harmed. For example the "friends of the Mississippi River" can bring a case against the local government for pollution of the river based on the harm that they can no longer peacefully enjoy the river. Or a gay couple can bring a case when their marriage license is denied.So, the basis is that the denial is invalid? Based on a lack of authority to deny for that reason?
It's supposed to prevent the S court from acting legislatively and simply adjusting laws, by now a days standing is hilariously easy to obtain, like the "friends of the Mississippi" example.
You could have showed your particularized harm by submitting a form 1 for a MG or a form 4 for a post 86 Mg. once denied you can show you have harm or standing. It never had to be approved and then withdrawn, simply denied at any point.
Their case is not to overturn the mg ban though, simply to state that it does not apply to trusts, only individuals.
*not legal advice