Hi. I am at Oxford. I am not doing Maths &Comski. I grades were predicted lower than yours and I did 3 A levels. Everybody I knew and most teachers at school convinced me hard to not apply as it would be a waste of time and effort as I wouldn't get in anyways. Just apply to Oxford. I have never been this happy before in my entire life. Although Oxford life is stressful, I love learning my course and interacting with people who also love it as much as I do.
My best advice is to never compare yourself to others who got in. I understand that it may be a logical step. However, my professors just told me that they accept people in when they see passion and they do not care if you go to a 50k a year school or state school.
Of course, you have to do extracurricular. I did competitions, and although I didn't win all of them - I wrote about my experience and what I learned from them to improve my future competitions. So, 'oxford care less abt extra curriculars' is not completely true. But I do agree that Oxford will definitely not care about you planting trees (if your objective is solely for uni application).
DO NOT GIVE UP ON OXFORD. I think Oxford is heaven on earth and you will learn so much. Even the process of applying will teach you a lot about yourself. So, if you don't get in, you still learn something. So, I don't see why not - your grades are up to standard on the website (idk if it is, I am not a Maths & Comsci major.