Are the A-levels I did over a decade ago really that relevant to my subject knowledge now, though? If someone got poor A-levels, due to personal circumstances at the time, for example, does that mean they can't be a good teacher?
FWIW I got As at GCSE, A and C at A-level and a 2.1 degree in what I'd consider my "specialism", but I've also taught GCSE Physics, and my highest qualification in that is a B at GCSE. However, I do have the skills to teach myself the spec. I've also in the past taught A-level Psychology (which was my other A-level), which I got an A in, but don't have any higher qualifications than that!
Stretching the brightest students is often not solely down to subject knowledge, although good subject knowledge can help interest students of all abilities. But often the stretch is thinking up activities that ask them to apply their knowledge, or synoptic activities which link together lots of different topics, or analysis tasks etc.
There are also things on the biology syllabus (e.g. relating to genetics) which I didn't learn at GCSE or A-level, the syllabus changes as new research is done and we gain greater understanding in some areas. Therefore, I do think it's important to keep reading widely around my subject and keep my subject knowledge up to date. In 20-30 years time, it's very likely some of what I learnt on my degree will be seen as out of date.