I am doing 12 GCSEs and expecting only A*s and As.
I do not know with much certainty what I want to study at university, although I am duelling between a science-based career or one of the business/economic type. Or both.
You must be certain of what you want to study at university if you are to choose your A level subjects. Because universities prefer you to have some background knowledge of the subject that you are going to study (from A level).
If you want to study science, you should be taking Physics and Biology. Do consider properly if chemistry is really needed for you. Because, say, if you want to be a biotechnology related doctor/technician, chemistry is important. But for an engineer, chemistry may not be so important. Same goes for biology - choose it only if you want to be a doctor/related profession. Physics is necessary for Engineering.
Mathematics is always a plus. Further maths will be needed if you want to be an engineer, otherwise not necessary. But do try to keep a mathematics subject. And be aware that further maths is quite a difficult subject. But you can do fine if you try hard.
For a career related to Business and finance, you should take Business studies, economics. Subjects like applied ICT or computing will give you an IT edge as well. But be aware that Business studies is not as easy as it seems - the case studies are very challenging esp. on A2. Economics is very easy at AS, but quite difficult at A2. I know because I've done them all.
You can take accounting too.
You should take maths, especially if you want to study the finance stream. Further maths is a plus as well, but not an absolute requirement.
Spanish (For which I don't have to study for)
Don;t think that you "will not have to study". My native language is Bangla. But I still had to study quite a lot for my Bengali O level exam because I was unaware of the different grammar constructs.
Business studies is useless and Unis dont consider to be a proper subject.Economics would be a better choice.
With all due respect, it is a very good choice for anyone who wishes to pursue a career in the field of business. But like all other subjects, it is not so necessary/useless if you wish to pursue a career in some other field.