Well I am not from a UK top university but a top Irish one (which actually doesnt really rank in comparison to Oxbridge etc) But I have had multiple offers at tier 1 I-Banks in london. I study economics & finance which is quite quantitative but not pure maths!
The reason why I got multiple offers is because (as far as I can see) I stood out due to my trading (with real money) and with that came initiative, decisiveness, enthusiasm etc
Your best bet in my opinion is to focus on maths as people said. If I was in your position and really wanted to be a trader I would try and get top marks all through your school, study economics and maths at Oxbridge, dont be a "nerd" and not have interpersonal skills, try and stand out with extra-cur activities ie set up a investment club, chair societies, captain of your golf, soccer, rugby team etc, DO set up your own real money account, and most of all enjoy yourself, dont ruin your unresponsible years!
I-Banks look for:
1) Teamwork
2) Leadership
3) Initiative
4) Motivation/enthusiasm
5) Quantitative skills
6) Problem solving
7) Activities outside of work/school
8) Humility
9) Poise
10) Someone who they like - can pass the airport test
Theres prob more......they've gone out of my head due to the weeks of interviews!!