I really can't bear this MN approach of 'everyone' leaving and there being no 'community' in London. Thousands upon thousands of kids go to state secondaries in London; quite a lot of them end up at university/in well paid jobs/not in gangs. It's just unbearably elitist thinking. I've seen people say there are 'no schools' in their area. It makes me so angry.
I live in SE London. There is one secondary I wouldn't want my kids to go to, because the gangs are out of control there, and it's never full, and everyone knows how awful it is. Genuinely it should just close, as the problem just isn't getting any better.
Other than that, I would consider sending my children to pretty much every other secondary in the area, depending on their needs. I don't much like some of the Harris schools from what I've seen of them because of their ethos, but it can be avoided, and it's not the end of the world.
In the time I've lived in my area (well over 20 years) I've seen the expectations and curriculum in some schools expand as the intake has become more middle class. I've seen schools be more explicit about countering gang activity, and succeed. I've see free schools set up dedicated to educating the middle class children who can't get into existing 'acceptable' schools. I've seen parents send their kids to private schools that aren't particularly good and end up taking them out again and sending them to state providers instead.
I have local friends whose children have gone from 3-18 in local state provision, many of whom I count as my community, who just get on with it. The local excellent primaries had very few children moving away because of secondary, although I think that's accelerated since Covid and remote working. And let's not forget all the people who just sodding live in SE London, always have, or for whom our secondaries are fantastic opportunities for their children compared to where they have lived themselves in the past.