Hi.
For A-levels, especially for Oxbridge, pick the so-called 'traditional' subjects. All of the things you are currently considering are traditional, so you've got no problem with that. Otherwise, you need to find a balance. So, pick whatever you are strong at, but also subject that could relate to your potential subject/field (although this is not necessary, as it is not lways possible). Maths and Further Maths are really good for any discipline, especially for Oxbridge and so are the sciences. Even if you are not applying for maths/science related degree, e.g. Law, PPE, Oxford and Cambridge really value Maths and Sciences, as it adds diversity to your curriculum and enables you to think logically. Also, they are quite hard subjects generally, and top universities like their potential students picking hard subjects.
However, I still think that the best thing you can do is pick whatever you are strong at. Nowadays, especially for Oxbridge it is more about your passion for subject (think about why do you want to do the subject you want to do) and performance at interviews. I am not saying that academics are not important, they REALLY ARE, but there are thousands of applications to Oxbridge each year with excellent grades across the board, therefore, a lot comes down to student's passion for subject and interview performance. As long as you can demonstrate your passion in your Personal Statement and in the interview (if you get one), you will be fine.
Moreover, A-levels are important, but to demonstrate you real passion, you should really do a lot outside the classroom - reading around your chosen subject, reading magazines... If you want to do economics, PPE, I would recommend subscribing to The Economist magazine - reading it would be the best possible preparation and will give you a headstart + you will not have to worry about wider reading when it comes to the application, so it would be less nervous for you, as, of course, you also need to concentrate on your A-levels. So if you seriously considering top universities, WIDER READING SHOULD START NOW.
In summary, as I said, it is not all about A-levels. Pick whatever you are strong at, but keep you options open, have a range of subjects (for example I do Maths and Physics, but also History, Spanish and Russian). Keep it diverse, do not try to restrict yourself, anything could be relevant. I would recommend doing 4 A-levels and drop to three in your last year if you need it. However, wider reading, all the books you've read in preparation for your chosen subject is as important as it would really show your interest OUTSIDE THE CLASSROOM for your subject and, ultimately would give you the better chance to succeed in the interview and have an application that would stand out.
P.S. The bits of advice above are more for Oxbridge than for anything else, but if you are seriously considering it, start preparing now, that would be my main advice.
If you have any further questions, do not hesitate to ask.