Wow. Lots to think about and respond to. Some random thoughts:
--of course there may be medical/SEN reasons a child is overweight despite the best efforts of the child's parents to feed the child a healthy diet; I don't think anyone is disputing that. I do think it is probably the case (though don't know how to verify) that a very large majority of child obesity cases do not involve such issues, and that if the "default" is that children are fed a simple healthy diet in portions that no more than satiate their physical hunger, there might be more empathy, and possibly support, for the outlying cases that involve special circumstances. There would certainly be a lot less "fat shaming".
--I have never understood how parents could not know when their children were overweight; even if they didn't find it odd that their 9 year old children needed 13 year-old trousers taken up it is a matter of literally 2 minutes to weigh and measure your child and input the numbers into the NHS's BMI calculator. I do this with my DCs at least once a year and just jot the date and measurements into an iPhone note (usually around the same time I take them for their school shoes). So far they've both always been in the "healthy" range (and the range for healthy is huge, unless I'm missing something in my grasp of statistics) so I don't note what their percentile is, though I generally remember roughly where it was.
--I am proud of the fact that my DCs eat a healthy, varied diet. Many may say it is just luck that made them that way - maybe, but I know for damn sure that if I hadn't fed them healthy food the likelihood they would find it and eat it on their own is nil.
--having said that, I'm certainly not perfect, and I know I have areas where I have backslid a bit and need to change. My DD10 was measured last week, and while she is still in the healthy range, her percentile has gone up to low 70s from 40s-50s in prior years. I have also noticed her ribs aren't as visible as they have been in the past. I may have hit her just before a growth spurt, but I do also know that I have been a little lazy this summer in letting her go for a walk with DS12 or friends to sweet shops where she has been spending her pocket money. I've been figuring she is getting fresh air and exercise and hoping that would offset the sweets but I've also been enabling a bad habit for her and DS. It isn't just their weight that is affected - their teeth too. Now that term will be starting soon we can put an end to those excursions but I also need to find a way to keep her occupied and give her independence without sweets being involved. It's my issue to sort. But I don't see any need otherwise to change her diet, and while I have told her the sweet jaunts are at an end, the main concern I mentioned was for her teeth (and to stop the sweet wrappers ending up under her bed), so no one needs to tell me to unclench and that I'm setting her up for an eating disorder.
--at the same time, I never fail to be amazed at the reaction people like Jamie Oliver get when they try to change people's attitudes towards feeding themselves and their families healthy food, and try to get the government to help create a supportive environment to bring that change about. People focus (so narrowly and stupidly, in my view) on his style that they won't listen to someone who actually does know what he is talking about - he is qualified now as a nutritionist. But no, they don't like his chipper, upbeat, matey style so they just turn him off.