Funnily enough I have Opinions.
KB should be aware of the research posted upthread that shows that girls in single sex schools are much more likely to take physics than in mixed schools suggesting that at least part of the problem isn't the 'hard maths' but the presence of boys (perhaps the knowledge that they'll be in a minority is off-putting, obviously one of the ways to tackle this is to try to ensure that more girls take physics so they're not massively outnumbered).
PP have rightly pointed out that girls have more choice at A-level due to better general performance at GCSE. The subjects where boys outperform girls at the top grades which would be needed for A-level are maths and physics...however the percentage difference in results isn't big enough to fully explain the lesser take-up of maths, and definitely not the far fewer girls taking physics.
For boys, it's more likely than girls that their good grades in maths and physics are their best results so they take them at A-level where girls might have several strong subjects.
BUT this doesn't mean that we should just shrug like KB and say 'I don't care if only 16% of girls take physics, let them choose.' She's the bloody social mobility tsar and should be well aware that a lot of the better paying jobs are in STEM. If she has a school stuffed with disadvantaged girls, then how about trying to improve social mobility by at least making some effort to improve their financial prospects by opening up the higher-paying job market to them?
I've found that confidence is a big issue with girls taking A-level maths. I've taught top set girls predicted a grade 9 who have needed a conversation about their suitability. They need someone to say 'of course you are good enough to take it' or to even just suggest it as a possibility for them. I expect the same goes for physics, particularly if it has a perception of being hard.
KB isn't doing girls any favours by reinforcing that perception.