What about Economics? Do you enjoy it, could you see yourself working in the city or in politics? With Economics you can combine it with so many things. I think you need to look at some sites on career advice and guidance or talk to your tutor at 6th form. They will probably remember others who did same A levels as you and what they went off to do.
The main thing is, do a subject you enjoy, or you will hate it and be wasting 28k in fees plus living costs! And when you have decided what you want to do further study in, look at the careers that it could lead to.
For example in my day, it was very common for Medicine, Pharmacy or Dentistry and Optometry to do A levels in Chemistry, Biology and Physics/ and or Maths. My friend got into Pharmacy with Chemistry, Maths and French! Some Unis will accept some unusual combinations as long as you have the main subject they want. For example everyone thinks or used to think for something like medicine or pharmacy where it is all about anatomy or treating disease you will need biology, but actually it's chemistry A level you need for both. You used to be able to get into both without biology!! With Maths and Economics have you considered Banking or Accountancy. And remember some of these companies will take you at 18 with A levels and train you and give you day release or night school to study their own material for their exams or the professional exams. And then you are likely ahead of the 3 yr uni graduates who may still need to do all the training that you have already done and you won't owe 28k in fees.
If I had my time again and had taken A level Maths with my A level Economics this is something I would have looked into. Do you really need to go to Uni? These days with 50% going rather than 20% 30 yrs ago, it doesn't really hold the cache it used to. Good luck