How do we demand it? A lot of people with physics degrees simply won't consider it for the pay (low compared to what they could earn elsewhere) and the stress (high compared to other roles available).
When I trained, there were about 8 physics teachers in my science cohort- that's with a £28k tax free incentive to train. I don't know where they all ended up, but of the ones I know, the last one left teaching has just taken a post in an international school, so she's leaving teaching in England.
The situation for physics teachers is so dire, it's often really difficult for them in their NQT/ECT years as they don't have a subject specialist mentor, and they end up with loads of exam classes. Often by their second year, they're expected to de facto lead their subject and produce resources for biology teachers teaching physics.
Because of the high turnover and the poor physics teaching in KS4, it's a less popular A-level and degree choice than the other sciences, and it's become a really vicious cycle.
I actually genuinely don't know how we solve the problem- it has been going on for years and years, and it may have got beyond solving.
But it won't be long before all the sciences go the same way, and other subjects too. Maths, in some ways, is in a similar state, but normally at least you're not the lone specialist in your department.
If you said all people teaching physics (at what level?) had to have a physics degree or physics PGCE, then you'd essentially be baring a number of schools in England from teaching physics.