I do not believe taking A level maths will help you gain admittance to a PGCE program. The entry requirements for Goldsmiths PGCE program in Mathematics are:
The extent of mathematical understanding you bring to the programme is important. You should have a degree in Mathematics or a related subject. Engineers can generally offer enough Mathematics, as can actuaries and physicists. Those with qualifications in Business, Accounting or Computing need to be able to show a substantial amount of Mathematics in their degree. It is also important that you have some knowledge and experience of UK secondary schools. Some applicants may be referred to a TDA subject enhancement course.
The amount of mathematics in a Biomedical Science degree can vary substantially, however, even the most mathematically rigours biomedical science course, probably is not enough. Further, many PGCE programs also require you to have obtained a 2.1 or higher. I think the combination of those two factors will put you at a substantial disadvantage. I think a Mathematics Enhancement Course would serve you much better, in terms of gaining admissions to a PGCE program, than A level maths.