When working out schools and where to live, you should look at TFLs journey planner. Easy public transport journeys are far more important than distance.
I also think it is worth looking holistically at what your son wants to get out of the next two years. Is it a major scholarship, or to settle well and to use the last two pre-teen years developing resilience and a sense of comfort within his own skin, or is he bright enough that he can manage both.
Newton rarely prepare kids for the Challenge unless they feel they have a good chance of suceeding. Other schools will have a scholarship stream where a whole class of boys move at a cracking pace. DS moved on to Westminster and I dont think entering via CE made one jot of difference. Indeed some of the more hot-housed kids had a real slump in the first set of exams as if they felt they had made it so could then relax.
There is an approach to parenting in Central London that almost has children as performing seals. The right school, the right position within the class, and lots of high level extra curricular, all forming the ideal background for a good application to the right Ivy. The right school, if you are not already in Colet or WUS, would be The Hall, Sussex House, Wetherby, DCPS if you are in South London, or one or two others.
I don't know. My instincts were that this was all too much both for DC and for me. Looking back I think those pre-teen years are very important and there is a lot to do beyond getting the grades. In his last two years DS used to read three books a week, about anything and everything. He spent his half term before CE at a cricket camp. Newton used to have his year group help with things like the nursery sports day, and it did him no harm to have both girls and quite a wide ability range (they split into two sets for teaching) instead of what might have been a stressful and hot housy experience. I think this chance to enjoy education has been really useful, not just in seeing him through the rest of his secondary years but also at University, where he has no problems with self-motivation and self study.
You will know your son and how far his current teaching is from the British system. But it is worth factoring in his wider development needs when considering what school might be right for him. Honestly we have seen some major car-crashes of boys who have found the pressure too much.
French was DS' weak subject, and the only one in which we gave him extra help. Happy to PM with details.