I'd personally advise looking to study Maths at the very least and Further Maths if you have the academic ability to do so. Economics is another very useful A-Level for going into a degree that is finance related and you can then decide a fourth subject of your choice. (You can choose something different to mix it up a bit. Universities like a student with a breadth of knowledge, and therefore having 4 mathematical/finance related subjects can sometimes be a bad thing)
Business Studies is also another viable A-Level to study but it described as a "soft" A-Level and Accounting can be useful too if you're interested in it, but I don't know much about it since my sixth form college does not offer it.