I would add to the helpful PP above, that you also need to be honest with yourself about what kind of parent you are.
I've had four kids go through Islington and Camden secondary schools, as well as all the various schools their friends have been to, and I know dc who have done really well (as in straight A* and Russell Group or Oxbridge uni kind of well) at just about all the local schools, including the ones that are not remotely on the fashionable middle-class radar.
Looking back at my dc and their peers, what is striking is that all the dc have done pretty much as well as you would have predicted for them at the age of 9, based on the child's ability and personality, regardless of the kinds of schools they went to - I know dc who went to really tough schools who did the straight A* and Oxbridge route, and others whose parents prepped them very hard for private or state selective who have come out with very average kinds of grades. One of my dc was at a school that lost its headteacher halfway through his time there, followed by a terrible Ofsted and teachers leaving in droves - it was not a nice experience and I wouldn't recommend it, but my dc's very average grades were entirely down to his lack of hard work, and plenty of his friends did very well indeed, despite the chaos.
So as long as you find a school which has a reasonable number of kids getting the kinds of grades you would envisage your child getting, it's probably safe to assume that your child can do well there too. But you need to be comfortable enough with your choice and have enough confidence in your child to ride out the inevitable ups and downs - if you are the kind of parent who in Reception can't resist checking other children's bookbags when they come for playdates, and who will panic and stress if the other dc is several reading bands higher than your own, then you will find many state secondaries (not just in Islington) will give you plenty of opportunities to twitch anxiously and stress over whether you should have gone private after all.
In the final analysis you just need to find a school where you think your child will be happy and where you can see that the teaching is good enough that your child can get the grades they are capable of. Open evenings are surprisingly revealing, and if you do the rounds in Y5 and then again in Y6, you should get a good steer for what schools appeal to you and which ones you find off-putting, and those things may not correlate with published exam results or with the popular m/c fashion.
It's really not as scary as it seems. 