Also, for the admissions test, I know there's a compulsory maths section, so has anyone got any advice for me, since I haven't taken maths, or anyone got any tips for what to expect??
I'm currently studying biology, chemistry and psychology, and got A's in all at AS (also did English Language and got a B)
The majority (like, 98%>) will have A-level or AS-level Maths or equivalent - in fact a majority of Physical NatSci, and a minority of Biological NatSci applicants will have Further Maths as well as Maths (although it's not nearly as useful or implicitly necessary for BioNatSci). This is your competition - consider it from their point of view, where they can take a student with untested mathematical ability on a course which uses maths fairly extensively in all course options to some degree, versus someone who has proven they can gain top grades in the subject, and potentially also take the extended course of study with Further Maths to boot.
While it is possible to study the Mathematical Biology route A without A-level Maths, you will be required to self study a number of topics before arriving, and you will take a reduced version of the course. While 2018 will be the first year to offer this route (as the former "no maths for BioNatSci" route has been removed now), it isn't a great stretch to imagine the majority would have at least AS-level Maths. Bear in mind the core Mathematical Biology course does require A-level Maths, and as above you will have to self study some topics as well as learn some of the content out of sequence through a parallel lecture series.
I would suggest you pick up AS-level Maths in Year 13 at the very least, both to help with the NSAA as well as to better prepare you for that course in particular, and in general, study and work as a scientist. Alternately you can take a year out and do A-level Maths while working/volunteering/other and apply while taking that. Regarding the NSAA, the relevant material is available on the University of Cambridge webpages, which I believe includes an example paper.
However Cambridge is not the be all and end all, and there are many other courses which may relate to your interests at other excellent universities which don't require or emphasize mathematical ability as much.