I'm sure China could find some judges to rule in the name of national security if it would give everyone warm fuzzies.
Judicial checks and balances only function when they're independent of the executive and parliament
A judge isn't a magistrate, but also: No, of course not. There are different layers of legal protections in the UK. You would be able to appeal the notice itself, you would be able to argue at the court against the decision, and you could make an appeal to a higher court if your were convicted. Furthermore you could make an official complaint about the investigation afterwards.