This is starting to sound like a state-school apologists' brainwashing drive. 
To counteract that impression, I would just say that I do feel for the OP. There is a definite tendency among certain groups of parents to portray certain fashionable schools as the only acceptable alternatives to private schools, and all non-fashionable schools as holding pens for feral illiterates with over-stretched teachers spending all their time acting as social workers. It's easy to feel that if you don't get it right, tehn your child will drop off the education conveyor-belt before it's even got started.
My dc are older now, only one is still in primary, one in KS3, one in sixth form and the oldest at university. All have beeen at state schools in areas reasonably close to N16, so I'm pretty versed in the politics of north London schools neurosis.
With the benefit of hindsight, I can say that 80% of the time spent fretting about dc's education was not time well spent. With each subsequent dc I have worried less, and it has not made any difference to the outcome. My oldest were in a primary school that did actually leave the brightest dc to coast, but three of that child's class are now at Oxbridge, so it didn't do any lasting damage. My younger are/have been at a primary with an outstanding rating but which is often not fully subscribed after the intial entry stage, because it's not considered one of the fashionable options (ie has a lot of dc off the north London estates and scores highly on all the indicators of deprivation). They have had teachers so good that I would have gladly paid to have them teach my child, and the outcomes for the more able dc are at least as good as the alternative schools with the micro-catchments of £1m+ houses.
My oldest dc went from the slack primary school to a super-selective (yes, with a little tutoring). She has done well, but tbh she was always going to be the child that did well, so I'm not convinced it has made any difference. For the other dc we hae gone the comprehensive route, and not ones that are insanely sought-after (roughly equivalent to Stoke Newington School and Highbury Grove, for local comparison). Their results are on a par with their primary-school classmates who went to more selective and private schools.
So I would say to the OP: mine down into the data for the schools you would likely be offered. The DFE website shows you the breakdown of higher, middle and lower-ability dc, and shows you the outcomes for each part of the cohort. It's quite eye-opening to see how some ridiculously sought-after schools are actually doing no better for each band of pupils than ones that are not on the m/c radar, and some are doing worse. Then go and visit, chat to the head, chat to the Y6 dc who show you round, talk to the teachers about what they find challenging and what they find rewarding about the school, and see if you like the vibe.
Obviously there are always parents who are committed to the high-intensity, high-status private school route, in which case fair enough (though I'm still not convinced the end result is that much different from what the same dc would have achieved in a more relaxed system). But the OP sounds as if she's panicking a bit, which is almost certainly not necessary and not the basis for a good decision. And if you really decide that you're not happy with the school you've been offered, then places do come up all the time in both state and private shcools, so the door has by no means shut if you don't get it right at age 4.
