Hi SNM,
I'm no longer in touch with her, but heard through the grapevine that although she was marginally out by 0.05 mile (or something like that), they still got a place. When I last spoke to her though, she was in a constant panic/sleepless nights etc., due to the change. Her elder son had attended there also; not sure if that helped, but know for sure that they weren't able to benefit from sibling policy either.
I'm struggling to pinpoint where you are exactly to recommend the best ones locally as I went through this just recently. I really don't envy you - it's a nightmare I know.
I'd agree on the St. Joseph's issue as in recent years I too have not heard good things about it.
From where I think you are, the best ones (in terms of results and caliber of children - sorry to sound snooty) are:
John Fisher
BTG
Harris Crystal Palace
Kingsdale (despite negative press, I don't feel they were doing anything different to what all the other schools were doing)
Archbishop Tennison
Wimbledon College
I don't know St Andrew's so can't comment on that one.
The list above, from my extensive research and I feel, general consensus, would be deemed your best choices. But once you've come up with your own list, I guess only then, you can decide what to do - move, have your ds travel, or otherwise.
You also mentioned you're a few years away, which means, there's also a chance that one that's not doing so well now may change dramatically in the future. For example, Thomas Moore's results suddenly shot up by something like 25% this year.
Another option you have not mentioned, is to possibly tutor you ds for a bursary place (assuming that paying full fee would be out of the question) for Trinity and Whitgift. They have tons of really generous bursaries where parents with earning of up to £75K can get help with fees. Just a thought, unless you're totally against private, of course.