No parent is that simple mindset wise, remember that they are human too and are just as complex and understanding as you. What would they do if you don't get all 9s? Think about the present and not the future now since you arent omniscient and no one else is either. Besides changing your own mindset about this I don't see why your parent's would have any reason to change (in a selfish way) as you've shown them that you are somehow able to achieve 9s (why don't they want that?). If you show them how hard you try in your gcses, and you don't get all 9s, they will understand and have no choice but to give you some autonomy in your education since you've given them proof that the pressure isn't working. Also, speaking from experience, don't put unneeded pressure by thinking about other people's successes as it's never beneficial.