I never knew what I wanted to do at school. In fact, I never really knew I was good at maths until my teachers suggested I try for Oxbridge. Anyone who is serious about reading maths at university will need to study maths and further maths at A level, but one of the keys to giving yourself flexibility in terms of your career path is what other subject(s) you choose to take with it. The traditional 'double maths and physics' combination is the usual option at A2 due to the overlap with mechanics, but if you are considering a career in business, finance or accounting, then economics and/or business studies would be a good choice. Alternatively, you might take the opportunity to study another 'standalone' subject that you enjoy, such as music or art, alongside double maths.
Once you start a maths degree course, you will begin to appreciate the great diversity of topics and applications that come under the maths umbrella. Depending on where you study, you may or may not have a lot of options to choose from, but even the most traditional maths courses, such as Oxbridge, provide you with a lot of flexibility as you move into the second and third years.
The most obvious career paths with a maths degree include academic research, school-teaching, banking & finance, accountancy, actuarial work, insurance, engineering, R&D and IT. When you are at university you will probably get a feeling for which of these directions interests you the most and choose your options accordingly. You may also decide to do a one-year postgraduate course, such as an MSc or a PGCE, to help make the transition to your chosen vocation. Bear in mind, though, that many professions such as accountancy or the actuarial profession will require several more years of exams before you become fully qualified.
The great thing about maths is that it teaches you to think logically and rigorously, and to question any assumptions that you make, so in that sense it is a subject that can be applied to almost any career where such qualities are important.