This is kind of an absurd question - provided you actually work to the same level as you have been as a baseline, and then rise to the level necessary for continuing through 6th form, there's no reason any subject would be an issue. Take the one you are actually interested in, and if applicable, that you may want to study at university.
I'd suggest taking Chemistry, in all honesty, as it opens far more options within the biosciences together with Biology, and is more useful background for going into the physical sciences. For other subjects that don't require any specific A-level background (or only require Maths, for example) such as Law or Economics, Chemistry is perfectly acceptable as well.