@Ferniebrook, there have been lots of helpful threads about Maths A level, combinations with Chemistry and mixing STEM and non-STEM A levels. To summarise (hopefully correctly):
Embarking on Maths A level with less than 8 or 9 at GCSE is cautioned against and some sixth forms or colleges have high GCSE grade requs for that reason.
Only a small number of Chem degrees require Maths A level (Oxford, Cambridge (natsci route), ICL, Durham, Scottish unis); most do prefer a second science, which could be biology rather than maths (physics without maths is not advised); psychology and compsci can be regarded as a second science by some unis (but not all).
Mixing STEM and non-STEM, where Maths is not taken, typically means two STEM and one non-STEM to keep STEM doors truly open. An alternative is to have all non-STEM but supported by core maths and/or more quantitative social science A levels such as Geog or Econ in the mix; whilst this probably means STEM doors are closed, quantitative skills are kept refreshed.
This is obviously a broad brush summary and not some set of immutable rules but might help when looking at subject options and the mixing of options across the various sixth forms.
I have DC who did a mix of STEM and non-STEM and one who was keen on Chemistry, hence the awareness and interest. They went to the sixth form in their comp and liked the general culture (some boundaries but no uniform), the fact is was not single sex (reflecting society at large) and the willingness of the school to really make option blocks work for everyone.