Thing is, without coming across as 'harsh', unless you are deemed selective and go to grammar in Y7 or transfer across in Y12, they are virtually impossible to get into. The waiting lists are long.
Y8 is a possible move on the 12+ but in reality if a child has been in thenon-selective system, even after a year, they are that far behind their grammar counterparts it is very difficult to catch up. Even in Y12, non-selective kids that have done amazingly well at GCSE find it incrediblly difficult to transfer into grammar 6th forms -they are 2 years behind, the ethos and work ethic is so different.Most of the grammar kids have started at least one A level in Y10, and have 2 or 3 AS's under their belt upon entering Y12.
I have children in both system, and I cannot labour the point enough about the volume of work and self discipline needed.
Vertical learning is (as I understand it) an American system that a few schools are trying. Basically you move up (or down) according to ability, not age. I doubt many schools would ever go that route in this country because of timetabling issues, isolation from year group, emotional immaturity, bullying. There are a couple of schools in Kent that have piloted it, whether it's successful and they have retained it, I don't know.
In all honesty, there are other issues I would address first. Area where you want to live would be the main one, and look at schools locally. Ofcourse schools are 'better' in affluent areas, simply because the parents tend to keep their children on task. If you are that sort of parent, your child will succeed.
Your Elder son is 16 in November. Schools tend to let their pupils go in May for "study leave" - I would suggest you start looking at 6th forms for him ready for next September. I know Beths take an extra 40-50 in y12 - but competition is hot for those places. They base theirs on the sibling rule, first.
I've been looking at schools for my Y6 - and Bexleyheath Academy is ticking all the boxes. It has just converted to Academy, has a massive injection of funding, and a very straight talking pushy Head. It's a massive school, 300 in each year group so the setting is very good - at the top end these children will have only missed their 11+ by a few marks. It is undersubscribed as you go up the year groups as it had a pretty bad reputation a few years ago. All schools go in cycle and my guess Bexleyheath is on the up. It will be the nonselective school of choice for the foreseeable future. They also practice the 'condensed KS3', meaning GCSEs start in earnest in Y9 and are spread over 3 years rather than 2.