I haven’t read all the replies but here are some thoughts.
I went to a comprehensive school that had a very wide socioeconomic mix and I found the disruptive pupils annoying when I was aged 12-13. After that we were streamed and they made no difference to life in the classroom but there was a bit of bullying at break times. At age 16 they all left and it was fine. I have no idea what my school’s stats were for exam results but I could guess that over half didn’t do well enough to even consider staying on.
The other half of us who stayed mostly got into excellent universities and did extremely well in life. I’m sorry to say that we all had similar family backgrounds in terms of parental levels of education and the part of town where we lived. The ones who left came from a different part of town and had a more deprived background. I can’t think of many who successfully crossed that divide. Importantly, I am sure that the naturally intelligent but socially disadvantaged kids did worse than the privileged ones who were actually not as bright as they were. Mostly due to parental support.
We all had the same teachers and the same school facilities. Looking back, I realise that the teachers/SLT maybe weren’t as good as my rose-tinted memories suggest because they clearly did not do enough for those other kids. We were the easy ones.
In your shoes I think I’d be mainly interested in the quality of the school’s facilities (which is too early to say as you are looking ahead ten years) and whether they are able to retain good teachers who will inspire bright children, and most importantly trying to get a handle on the classroom experience- do they stream, do they offer academic subjects, do they control bullying?
What I am trying to say, I think, is that the school results may not really mean much and if your kids are academic they’ll probably be fine there.
On the other hand if they are not academic then paying for private will probably be the right thing to do but you won’t know that for a long time, and also you’d need to find a non-selective private.
Of course you could also pay for private for academic kid if you want all the lovely facilities and all the bells and whistles. That’s what we do, but we are in a very comfortable financial position with only one child and two high incomes. Our decision is based 100% on quality of school experience, not academics and we see it as a luxury.
Grammar area is a non-starter as you say, because if they do not get in they will end up in a school with an overall higher number of kids who do not want to learn.
TL:DR privileged bright kids will do well in any comp so don’t get too hung up on the rating, forget grammar, pay for private if you want a luxury school experience.