I did Maths Chemistry Biology ICT at AS and then dropped Biology and ICT and picked up Further Maths and self studied it in the A2 year alongside my taught Maths and Chemistry. I think this helped me in my interview as it showed the independence of my learning.
Then I went to University of Nottingham and did BSc Mathematics. They do a couple of modules which have some massive crossover with CT (stochastic modelling is a big help!)
Honestly I would do a Maths degree because wherever you go they will allow you to shape the degree towards a certain career direction, meaning you can take some advanced probability and statistics modules which would offer you 1-2 exemptions and a good foundation for any career in finance!