Having read the full thread now, I really suggest you take professional advice. We’ve used Gabbitas before and we are very happy with them.
One of our sons also started school late, for similar reasons (we don’t speak English at home and he didn’t have the right tools at 4 to start school). But when he did start, he went straight into the “right” year - partly to avoid confusion at the 13+ stage.
So if your son started two years later, he should have started straight into Y2, and if your prep advised you to start from Reception instead, I’d stop taking their advice immediately.
And, if he’s two years older than his classmates, how good a grasp does the teachers really have on his ability for his age? Of course he’ll seem “very intelligent” compared to other children in the same year, he’ll have two extra years of maturity.
You have two options: Either keep him in year and have him start secondary in Y9, knowing that it will take a lot of the schools out of the race (neither of those you mention will accept boys his age into Y9. For Westminster they have a strict 12 month age bracket for the kids starting).
Or you can tutor him intensely to catch him up on Y6 and Y7 curriculum to have him sit 13+ at the right age. Most schools will accept this, although you do need to speak to the admissions office as he’ll already have missed the pre tests. Your prep head should help you with this, but as they seem to have been useless so far, you might want to book meetings yourself and get selling...
It’s a tough situation, and it’s absolutely worth hiring an Education Specialist to help you sort it. Like, now!