Anyone applying to a reputable university for something maths-y or physics-y will clearly be able to do science, and if you can do 3 sciences then you can almost certainly do 4. Consider doing something to set yourself apart - I took maths, further maths, physics, classical civilisation and art (n.b. by "something to set yourself apart" I do NOT mean general studies, film studies, media studies etc!). Some people take an essay subject as a fourth subject for AS and then drop it at A2, to show they're 'well rounded' and all that: something to consider perhaps...
Consider this:
How many people applying to study a maths/physics related subject (assuming you are) at university will be taking maths/F maths, physics and chemistry, or economics? Can you distinguish yourself between other candidates with the same choices? If you're confident you can then go right on into whatever subject you're happiest with, but if not you need to start considering how to get yourself noticed by universities...