It is a perfect storm - in the Uk. I live in France... of the 60-odd kids in my ds1's year at primary, I can think of 1 who is overweight. When I come back to Scotland for a visit, I am taken aback by how many overweight kids (and adults) I see around my local town. So YANBU to notice it: it's true.
Having lived outside it for some years it looks like the UK has a food culture designed to create obesity e.g thinking of junky, calorie-intense food like curries, fish suppers, pizza etc as a 'treat' rather than seeing good quality food as a treat. Prioritising cheapness and quantity over quality every time.
In terms of school meals... my kids get a salad starter, a main of meat / fish plus a small side dish, cheese, then yoghurt or seasonal fruit. Chips maybe 3 times a year. Cake / patisserie for dessert once a week. No options, no choices, packed lunches not allowed. And no snacks - play pieces are unheard of here. Very strict rules of not snacking - one snack a day at 4pm and that is it. No morning tea, no bedtime supper.
We ate out in a popular family restaurant with my PIL last time we were back in the UK. For a start, the venue does not exist here: a huge out-of-town site, with a whole range of restaurants, all chains, all selling cheap rubbish as a ´treat'. Frankie and bennys, Harry Ramsden, Brewers Fayre, etc etc. The one we went to had endless free refills of fizzy, sugary juice. The standard glasses of wine were at least twice a normal measure - and the large was huge. The food was... cheap and cheerful was the best thing you could say about it. My FIL ordered this dish.., it was a butterflied chicken breast, smothered in artificial BBQ sauce and grilled cheese, served with a mountain of chips and a tiny salad on the side. There were more calories in that one meal than any human being would need for a week, never mind one meal. The whole menu was like that. And the place was mobbed - as was every other one of the chain places - on a weekday night. Full of people 'treating' themselves to the equivalent of that BBQ chicken thing.
AA Gill wrote a really good article about how rationing affected British cuisine, which was once just as good as that of other nations. In France, a cook was - and continues to be - judged by the quality of his / her 'board'. British housewives OTOH came to be judged by their ability to ´make do', to turn crappy, cheap, low quality ingredients into edible food, thus diminishing the value that we (Brits) place on quality over quantity. So Brits today value large amounts of cheap food over smaller amounts of good quality food every time. And we eat mountains of it, because it's cheap, addictive and ultimately unsatisfactory.