Most primaries have guidelines. Just ask.
Most are sensible.
Ours is : fruit, veggies, cheese, crackers, breadsticks add for snack. Pretty much anything except sweets, chocolate, crisps. In KS1 the snack is given out and is always a fruit or veggie selection, sometimes cheese too, a small matchbox sized portion.
Lunch guidelines are basically anything but nuts.
If a child in my class repeatedly brings in two bags of crisps a day for lunch and snack, I will ring home and ask that the snack be healthy.
But to be honest, this is not a big issue. Far more worrying to me is the amount of kids that turn up to school hungry, with no snack, or inadequate lunch. Our school shared leftover ks1 snacks with ks2 children who are hungry, and we've started a discretionary school meal for kids whose lunchboxes are inadequate. Free school meals don't reach the low paid.
That's a real crime, not the worry about a bit of cheese nutrition. Kids in need have rocketed over this last year in my area.