Busybodies always seem to assume that, if forbidden to do the things they don't like, you will do the things they favour. As that's an obvious fallacy they will move from ban to ban until everything of which they disapprove is forbidden and we have the totalitarian society they crave.
If the "prolonged screen time" of which "Public Health England" is so disapproving were in front of a Kindle for example, reading with the intensity that I read when a young boy, would that be a problem? I managed to spend many more hours a week reading than modern children spend watching television, chatting on Facebook and playing video games combined. Yet I got exercise too because I walked or rode everywhere on my bike, was bought a season ticket to the local swimming pool as my birthday present every year and - most importantly - was allowed out on my own from a young age. I took all my exercise unsupervised, not just going to the swimming pool every night on my way home from school, but out and about on the streets and in the fields and woods with my friends.
Modern children are not the problem. Modern parents - with their imagined terror of what might happen if their children were free range - are the problem. Children are not getting enough exercise because many respectable parents are too busy to take it with them and too paranoid to let them take it on their own. Is it any wonder they spend a lot of time with electronic entertainments? Those, after all, are available in the home, without parents making a ludicrous fuss. Is it any wonder they chat with their friends online, when they are not allowed to be with them?
There may even be a positive side to all this. I suspect, for example, that one could correlate the decrease in violent street crime with the increased popularity of computer games. I happened to wander into a video games store in my home town on a working day and was amazed by its pale denizens who looked as though they lived under a stone. They reminded me of the local youths of my day who used to get a lot of healthy exercise chasing and offering violence to respectable, studious young people. I am sure their parents have no more heard of "play dates" than their grandparents had, so without the efforts of Electronic Arts et al., they would certainly be out unsupervised. Maybe they are not better off spreading their waistlines and satisfying their lust for violence in front of a monitor, but the rest of us are. If respectable, paranoid parents allowed their children out unsupervised, they would probably be safer now than in the "good old days" because the nasty kids would still be indoors - killing each other online. If only the computer industry could make cyber sex as satisfying as cyber violence, maybe those families would also stop breeding?
More seriously, if this now falls under the definition of "public health" in Britain, there is no hope of freedom. The state took its public health powers in order to clean up the disease-ridden slums and restrict the activities of the likes of Typhoid Mary - because those were external threats. No-one could reasonably object to such powers but, as always, the people to whom they are granted never stop trying to expand them. They do so in their own financial self-interest and because the people attracted to such jobs are those who like to boss others about. However sad it may be that children are getting fat because their parents don't let them get enough exercise, it's not a threat to anyone but themselves. Therefore it's not the state's business. A country that has the resources to employ expensive professionals to carry out studies of what its children do with their recreation time does not know the meaning either of "austerity" or "common sense".