I am sorry but it seems to me that you regard state school as a social experiment for a few years for them to get to know the kids "below stairs" rather than a valid education in itself.
To a point you are right about friends, but the friends they make in the early years wont matter much. My friend was moaning that her child was displaying a rather brattish attitude berating children without swimming pools at home, so she decided to speak to a mum in her sons class at prep what SHE did to teach her children appreciation for their good fortune, and got the reply that the prayed together reinforcing that it was Gods will that they were so fortunate.
Having said that. My oldest is going into the independent sector for Y7. He sat the exams for various public schools in January, and got offers from the schools he wanted most. He has a state background, which probably is worse than your average UK state education. He was educated largely in Norway.
He did Reception and Y1 in a London RC primary, then we moved to Norway where they start school at 6, so he started a fresh, and spent 3 years in Norway. He were still just doing addition and subtraction and basic reading homework when we returned to London 3 years later, where he joined Y5. (from Norwegian Y3, thereby "skipping" Y4 alltogether)
He was shellshocked. He did have a tutor from January, for a year (not including holidays) until his exams, and still did really well.
He has not had music. He has not had Stagecoach (which the other parents proclaim is brilliant for learning public speaking), neither has he had French, or any other foreign language (not counting Norwegian which would not count at all)
What has he done? He has done lots and lots of mountaineering and outdoors survival skills. To be honest, I bet he has done the equivalent of the Duke of Edinburgh Gold award, purely from a lifestyle of hiking through mountains and managing on his own. He can find shelter, he knows how to source what he needs from building a bonfire from scratch. He knows how to handle himself with a box of matches and a knife. He can put live bait on a fishing hook and go for it. He can navigate using map and compass. He can build robotics, and program the robotic brain of lego mindstorm, he is fabulous at skiing, mountain biking and has stamina like an ox. He turned 11 on saturday and has presented us with a shopping list with all the requirements for building his own computer. He is pretty great. His education up to y5 has been abysmal, but he has still earned a good place at a private secondary, because of who he is, not where he was educated (rather in spite of where he was educated)
So, my advice to you, enjoy your children, challenge them, nurture him and her, but dont think that school is alfa omega for making them who they are, YOU are.