What other A Level subjects would be useful towards applying to do a Mathematics degree?

To compliment Mathematics, Further Mathematics would be an ideal subject to take. It is not essential, however the additional practice would benefit your understanding of Mathematics, easing you into even higher education. Other subjects in the Sciences, such as Physics and Chemistry, may also be useful as they provide you with knowledge into applying what you have learnt from Mathematics, adjusting your way of thinking to be more practical and to further your understanding.