Be very careful when looking at the the league table results. Yes, Bancrofts beats Forest each year. I believe that it used to back when PartyPooperz was there, although league tables weren't published. BUT Forest always punches above its weight for Oxbridge/London/medical school results. Teachers were dedicated to the (long-term) success of their pupils as adults, not just exam-result producers, and they know how to work the system to the advantage of all of their pupils, not just the ones who are already in the know. Check the destination lists carefully as essentially this is what Forest is interested in - getting their pupils into the best place to continue their education.
I left there quite a while ago - a number of my teachers are still there, or still have strong connections, which I feel speaks volumes about the place. I'm stunned that you manage to come away from the impression that a bright child would ever be held back. Given that I was taught by the some teachers who are there now, I can't believe that they wouldn't continue to let children fly, by whatever means possible. They always had a top set which either sat all their exams at an accelerated rate, or simply did more subjects, or more at AS early etc.
FWIW I would have probably let the league table results down in my day. I got a D in my 5th A level. But as I had my EE offer from Oxford, I didn't really care. Bancrofts would have probably not let me sit the final A level as a school candidiate (always worth checking the number of private A level entries ;)).
Liz Sidwell, current Schools Commissioner was there for some years, and both of her children went there. Obviously historic, but again, many of the heads of department were there in her day.
Again this is just a view of the senior school. The prep school will have changed from my time.