One law?
I’d introduce a mandatory physical education and health literacy law for every child from primary school through age 18.
Not optional. Not “one PE lesson a week.”
Structured strength training, cardiovascular conditioning, nutrition education, mental health literacy, sleep science, all taught properly.
Why?

Because most of society’s biggest problems trace back to preventable lifestyle failure:
Obesity
Type 2 diabetes
Cardiovascular disease
Depression and anxiety
Low resilience
Poor self-esteem
Reduced productivity
We spend billions treating symptoms instead of building durable humans early.
If every child learned:
How to lift safely
How to run efficiently
How to fuel properly
How stress works physiologically
How to regulate emotion through movement

You’d see:
Lower NHS burden
Better mental health outcomes
Higher workplace performance
Stronger communities
More disciplined adults

This isn’t about aesthetics. It’s infrastructure for human capacity.
Most adults are trying to fix at 35 what should’ve been installed at 12.
Build strong kids → you don’t need to repair broken adults.
That’s the leverage point.