I have been to a gazillion Oxbridge talks at my school and a couple hosted by Oxford ambassadors and they place so much stress on the fact that they couldn't really care less if you're doing D of E, helping at girl guides, unless these contribute to your Mathematical skill (in your case), and for degrees such as history, your research skills etc. I wanted to study history initially, and for that, extra curricular (or super curricular as they like to call activities relating to your degree) is pretty easy, do some essay competitions, bit of extra readinh, subscribe to magazines.
Now, I am in your position, same thought process, and am thinking of still enetering essay competitions, writing about maths since it is very philosophical at times. Express your interest for the weird and wonderful things in the world of maths. I would say tutor some maths, but that doesnt show your interest imo, maybe help at maths workshops that are a little more wacky and interesting. Luckily, you do fuurther maths, so your understandng is already quite detailed and perceptive i am assuming.
Either way, regarding your initial concern, you have a higher chance of getting in than I, since I am only going to d AS further maths next year because stupid me didn't choose it earlier, and i screwed my C2 exam when I had a panic attack so... possibly a gap year to do a2 further maths and further additional for me, hopefully then, oxford or warwick will want me