If I do a two year chemistry PGCE with a biomedical sciences degree, can I teach both Chem and Bio at A level?

If you have a secondary PGCE then you are qualified to teach in a secondary school with no limitations on the subject - in theory you could teach German. There is a perennial shortage of Chemistry and Physics teachers and lots of the ones currently teaching it did not start out that way, so yes, if the school was happy with your subject knowledge (or just desperate) they could get you to teach A level Chemistry.