You are still overthinking it. Look at the nursery and pre-prep options. Very young children are surprisingly different. Some thrive in the sort of busy and boisterous setting, which might crush a quieter more sensitive child. Go by feel, go by which environment is most likely to get your child ready for education in the broader sense, which includes curiosity, emotional and social skills and an interest in learning.
At every stage I think it is more an issue of keeping doors open. A prep school which regularly has a good range of destination schools, so the most academically able go to the super-selective schools, but a good cohort go to the DCs, Emanuels, Radleys, Sevenoaks, etc and a few to less selective schools like Shiplake, Frencham or St Dunstans.
The real cost of a long commute is that your son will have less time. Less time to enrol in after-school football (great at teaching teamwork and resilience) or explore interests. DS, via a computer game, became fascinated with Ghengis Khan and aged 10 happily explained to SPS during his interview that he wanted to be an archaeologist in Mongolia because he liked digging and liked history. He got a place, and they explained that they were interested in boys with curiosity who pursued their interests.
The process is not just about brand name schools/universities, marks achieved and ticking boxes. Talk to any City recruiter and they will moan about kids who have won the glittering prizes who arrive entitled rather than being willing to muck in. And read a few linked-In profiles. People who arrive via the longer, scenic route often have more skills in place.
In short, look at local options for your child and find the right one. Don't rule out state if you have a lovely state primary down your road. Do then have a go at 7/8+ options if your child is ready and up for the challenge. Schools like Dulwich Prep can offer a better 7-13+ experience than WUS or Colet (whatever it is called now.) The latter two will have a number of quite anxious, quite pressured children, and can be quite competitive places. And from observation, if a child has not been happy there aged 7, their peers' view of them carries through till the age of 18, with no chance of a fresh start.
If you get a 3/4+ place at Dulwich Prep, consider moving. Or if not think carefully who does the school run. The pre and after school times of the day are so important. If not try to have a parent do both journeys, and use the time to talk properly to your child. You parent, schools educate. Both are important.
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