The private school reckon DS is 2 years behind their curriculum. We've been told in state that he's meeting expectations. How is this remotely acceptable?
But there will be lots of children in state schools who are doing much more than just "meeting expectations", and there will be plenty of kids at your ds's school who are probably working at similar levels to the private school.
Private schools tend to be selective. Even those that are not explicitly selective will be selective to a significant degree because of the kind of families that can afford private education and choose to pay for it. Many are also quite good at moving children on when they can't keep up.
So if the majority of kids in a particular private school are mostly working ahead of age-related expectations, that doesn't necessarily mean that the school is doing a better job of teaching them, or indeed that any individual child transferring to that school will suddenly start doing better. The chances are, most of the kids with similar demographic characteristics in state schools are also performing significantly above age-related expectations.
Of course, some kids will benefit from slightly smaller class sizes/pushier academic environments. I think this is particularly the case for those kids who hover just above average academically (ie fairly bright but not high fliers) and/or the ones who lack drive/self motivation. Private schools may well offer more support to help those kids succeed. A smaller number of private schools may also offer great support for kids with SEN (though this really depends on the school... sometimes, they will get much better support in the state sector.) For the kind of kids who are likely to excel anywhere, I think it's a waste of money, personally.
And of course, some people are happy to pay for fancy buildings/one-stop-shop extracurricular activities so that they don't have to facilitate these out of school etc. Fair enough if you think it's worth it. I didn't really care about buildings and preferred dd to do extracurricular stuff outside of school in any case, so those things weren't really selling points for me.
Ultimately, I think it depends on the schools in question, your individual child(ren) and what you're looking for from a school. I think you're working on seriously flawed logic though, if you're forming an opinion that private = good and state = bad on the basis of a one-day marketing exercise carried out by one single school. Hats off to them though, as they've obviously got the marketing just right for their target audience!