Why are you desperate for coed?
Eltham college is an excellent school, although I agree that it will not prepare your DS for a grammar, if that's what you have in mind. Your DS will get a proper all round education, with lots of sports, music and so on. The year 4 national curriculum is taught in year 3, year 5 in year 4, and so on.
Breaside is one of those schools that crams prepares for 11+ for the Bromley and Bexley grammars (possibly Kent as well).
I don't know much about St. Christopher's.
You would be able to get a 4 bed house nearish to these schools for £500K.
I'm amazed that you say you're looking for a "top" independent school, and are looking at either of those two, though.
I was not impressed with Merton Court, incidentally, which has been mentioned, upthread. It's absolutely rubbish for any kind of SEN, and from about year 3 or 4, they start to pressure parents into paying for additional tuition, which is how they get their outstanding 11+ results.