For the purposes of this post I'm not so much looking at the various historical fantasies and alternate histories set in London, its more about fantasies set in modern or contemporary London.
Rivers of London by Ben Aaronovitch - 6/9
Neverwhere by Neil Gaiman - 6/9
A Madness of Angels by Kate Griffin - 7/9
Stray Souls by Kate Griffin - 8/9
King Rat by China Mieville - 7/9
Kraken by China Mieville - 8/9
The City's Son by Tom Pollock - 8/9
Sixty-One Nails by Mike Shevdon - 6/9
Some of these scores a bit rough as I'm scoring from memory, so it's possible a few of the above would actually score higher if I read them again.
I can think of other books I haven't got around to reading yet, and I expect there are even more examples I haven't even thought of. If you get a score for a book that I haven't mentioned here do let me know, especially if there's a book that gets a full house.