Just as a comparison, I live in a poorer country than the U.K.
Comparing the pictures of lunches here to what my son gets is pretty shocking. Considerably less healthy, and way way less vegetables and fruit. (if any?)
But it is more than just the lunches.. it is the whole attitude and system.
This is how it works here:
One parent (part of a parents council) is responsible for food communication. They receive the nutritional details, and ingredients (as a photo) of exactly what will be freshly cooked in the school that day. They then send them out to the social media groups of all the parents, so that every day you get to see exactly what your child is being offered.
Every child eats the same(allergies accounted for of course), and no pack-lunches allowed. It is a healthy nutritionally balanced hot meal,, and the whole class and teacher eat it together at the same time.
Every day a couple of students are the "lunch helpers" who help other children, and supervise the whole thing.
The students all clear up their plates/bowls, and then all go together to play/enjoy lunch after.
Imagine that in the U.K.?
The basic expectation that every child should get a free and fully healthy meal, where the parents know exactly what is going on, and the culture of eating healthily and together is a fundamental part of the school day.
(Of course it doesn't work if you have a lot of fussy children though....)