Maths, Further maths and Physics is classic trio that would allow you to get into most maths/computer science uni courses. Physics is especially good as both maths and further maths have mechanics module so you would have advantage over other people doing it. Economics is like a cherry on a cake - perfect addition, as employers would love to see somebody who didnt just do maths, but who is familiar with economics aswell. so imo these are amazing subjects for you!