I don't know what your question is - you cannot do FM now so what is the point if you are going to apply anyway. As many have said already, Oxford were biased, as you claim, they would 'require' FM. They don't and you already know that.
This is the problem - too many people obtain the highest grades now and cannot gauge themselves against others. Although confidence is a good thing, I would not claim you are 'naturally good at' something until you have been tested and have lived in the outside world a bit more. A bit like saying running a marathon is easy - you can say that when you've done it.
The exams were much easier than when your uni recruiter took them, and 'not revising' is a particularly subjective comment - homework is revising, classwork is revising.Maybe in your parent's time it was, but a 9 at GCSE nowadays is no big deal, some of the GCSE population have just walked in from Syria and Somalia and haven't had it so easy. Others have taken it a year early, with extra GCSE's. Without Further Maths, you don't really have an opportunity to show you are good at maths so just accept it and do well in the subjects you have chosen because you are a long way off from being an Oxford student just because you want to be one.
It is useless trying to speculate on 650 or 850 ish blabla, you are an individual they are individuals and frankly nobody knows what you or they had to offer in reality.
Some people make threads because they like to hear confirmation that they personally are Oxford material and have a chance - but everyone here doesn't know you from Adam.
Just prepare for the tests like everyone else and stop being distracted about things you cannot change.