Greycoats is a girls' school, so not much use to teh OP!
Depends what you're after, really, you need to narrow it down a bit. There's such a huge variety in London schools: do you want good comprehensive? Academically selective? Mixed or single-sex? Relaxed or formal? Religious or non?
For selective you need to think about SE London -- Bexley, Bromley, Kingston or Sutton.
There are loads of good comprehensives scattered about, but you'd need to move into the right area. Fortismere in Muswell Hill is the prototype of the desirable comprehensive, but you do have to live on the doorstep and it's expensive around there.
Religious schools are generally less fussy about where you live, as long as you meet the criteria.
You really need to start looking around, as you may find that the schools everyone raves about don't do it for you -- we went to see the London Oratory, which is regarded as the Holy Grail for many, but we really loathed the place. Which is a bummer in many ways, as ds would probably get in there, but best to know beforehand. Horses for courses.
Have you tried subscribing to teh Good Schools Guide? Annoying in many ways, but quite good at giving you a flavour of the schools. No substitute for actually visiting, though.