Definitely check college websites as they may have their own subject requirements.
You will have trouble in the NSAA exam because it's split into maths (which everyone has to do), then you pick two out of physics, chemistry, biology and advanced maths & physics. That's in the multiple choice section; for the second section you'd be fine, you could just do both chemistry questions.
It would probably be easier for you to learn the necessary physics and maths for the advanced maths & physics questions rather than learn A-level biology from scratch.