I think for certain maths would be a good help as would something like geology. I think also if not a subject at all, just reading the right sort of books would go a very long way. I recommend things like The Selfish Gene, Darwins Dangerous Idea.
The logical thinking in maths would guide you very nicely in science. I guess it also depends on what area of science you want to go into. Science is an approach that you can apply to many different subjects/areas.
My research is on language and voices, and in my lab at present I have 4 researchers who did Biology, English Literature, Music and Psychology for their undergrad degrees! So there are many routes into a particular part of science, and they don’t always start in science subjects.
I think being a scientist is more to do with the way you think than the things you know. I also think it’s great to study non-science subjects alongside science at A Level, because it gives you a broader perspective.