Prosecute them for what, exactly?
If it were me, I would make it part of unfair dismissal laws. Make it so that a model agency cant fire a model for being too big unless they can prove that it would
be healthy for them to lose weight. (Im assuming a "creative dismissal" clause like we have in Australia, where reducing hours to make the position untenable, or similar tactics, count as trying to fire someone)
That's what this new rule accomplishes, it makes modeling agencies culpable for employing unhealthy models.
But it doesnt... It makes them culpable for employing models that have a statistic that has a vague correlation
to being unhealthy.
Your other concern is addressed by the fact that a healthy person with a BMI under 18.5 can model if simply given a doctor's note saying that they're healthy.
I disagree that being able to get a doctors note undoes the message being sent. On the one hand, the model is still being told "we are assuming you are unhealthy - if you want to work youll have to prove different", and potentially at their own expense and time. And on the other hand, it seems likely to me that youll simply get dodgy doctors that will give out health passes to any model that comes to them in the same way that here in Australia, where you can get a doctor's note for being sick off work, there are plenty of people who will tell you "Oh yeah, I know a doctor... just go to him, he'll give you a sick note", so the problem is unlikely to be addressed properly anyway.