"I'm not sure I agree with you Agapanthers I think a lot of parents at private school have jobs with long hours, such as doctors and lawyers. Many of them don't have the time to spend with their dc doing endless activities. Other parents, who are wealthy, perhaps through doing well in business, aren't academic and wouldn't dream of taking their dc to a museum. "
That's true, but I speak to my fellow parents and they are indeed signing the kids up for activities, and probably only half work long hours, and those that do have an au pair or nanny to drop off at ballet and so on
And working-class-done-good is a small minority (they do exist), most are in middle class professions.
"I don't think a fancy beach holiday at the end of every term is healthy for a child, academically speaking."
It's probably fairly marginal (e.g., my son's French has improved from our holidays, he has seen volcanoes in person for geography), overall, but all these marginal gains add up.
"On the other hand, parents who state educate their parents are often quit motivated to help their dc. Not all of them don't give a crap about their dc's school career."
Definitely not. But it varies from area to area. In a nicer area it might be just one or two, but those parents can certainly have a disproportionated negative influence. In a poorer area, negative choices are almost universal.
"I also wince when teachers use sn children as a reason for poorer standards than private education. Some parents who can afford it choose private for their children with sn (dyslexia etc). "
Well there are specialist private dyslexia schools. Mainstream private schools vary, some aren't interested in even minor special needs, whereas others will address them, but with the help one-to-one support directly charged to the parent. There aren't any that are open to anything, they would consider each child on an individual basis, and have the freedom to say 'this child is too much trouble', and walk away. The state ultimately is the insurance policy that has to take everyone.