It does make me laugh when I hear these comments about the 'royal schools' as having children at one and certainly not being people with holiday houses this is not our experience at all!! I would be wary of listening too much to generalisations of the parents. I suspect their are 'horrible' parents at all schools and to think that all parents are the same at a school is ridiculous. Our experience of the parents that have been in our classes is that they are remarkably down to earth and at no point have we been felt at all judged by the fact that we can't afford as much as other families might. In fact most of the families we know there have two working parents (ie they can not survive on one income alone).
That being said it is a huge financial commitment and I would agree with this part and if we did it all again and had decided on state school (which we do often wonder about) I think we would have stuck with the school we had started at - ie either done state the whole way through or private the whole way through. Going private was something we decided on the moment our children were born so we were saving and preparing for it from the start but it is still a huge financial commitment and the 5% increase every year is a killer!
However, I hear from friends who have children at good state schools that their children are doing just as well and you do sometimes wonder if it's worth the cost but then actually it is ironically the parents at our current school that have made it worthwhile in that we've made some great friends who are in no way 'ghastly' or 'seriously horrible', just hard-working people who are wanting the best for their children (even if the best is not necessarily what they think!)
I would just think about the financial commitment and if you can afford it go for it but if you do end up going to the state school I'm sure your child will have a great education too as we're lucky to have some fantastic state schools in this area and if you're in the catchment area that's amazing.
The only thing I would say that you do get at private is all the extra curricular opportunities but these come at a huge cost too (£35 for a half hour music lesson!) and as someone else said you can get these opportunities locally anyway at a much smaller cost.
The only other comment I hear is that it is hard to move from private to state once you've started...oh and apparently the robots are going to take all the jobs soon anyway so is it worth spending the money
Good luck!