The thing is though if everyone had such a nihilistic view of state education as this and given the opportunity went private, where does that leave the concept of education for all. It is all very well arguing that you are doing your best for your individual child but don't they have to live in this society that we create unless of course like Thatcher you think there is no such thing as society.
It's not nihilistic. It's just the reality of the system at the moment. I've worked in schools in a range of catchments and contexts. Anyone who thinks that the state system is a beacon of fairness and excellent provision and most kids will reach their potential anywhere is very naive.
Parents are understandably going to do what's right for their children. Why would any parent (with the means to do so) find out their child is without a maths teacher for GCSE, is likely not to achieve their potential and then say "actually DC I'm not going to get a tutor to help you learn"?
As I've said repeatedly, there's good state schools, failing state schools and everything in between. There's a thread at the moment where parents are sharing their DC's experiences in secondary school, though I believe the thread title/OP is more Key Stage 3. What needs to happen is people use their vote wisely, lobby their MPs, pay attention to the issues, not create silly boogiemen about what parents and children are like if they use private schools.
As with many issues, it's very easy to claim things are made on principle when you're in a privileged position to have a lot of options and are unlikely to ever be in the situations others face.
It's also silly to claim that on principle you're not willing to fund anything that disadvantages other students, but then go on to do tutoring, instrument lessons, sports coaching etc.
There's nothing wrong with any of the decisions above by the way. Many parents do it because the system is far, far from ideal, but trying to claim some moral high ground because buying educational opportunities stops short of private school fees makes no sense.