Here’s a post by Deborah Hewitt at tor.com: Time-Hopping Across 5 Fantasy Londons
She mentions five fictional Londons, starting with the layered 1819 Londons of A Darker Shade of Magic by V.E. Schwab: Grey London, with plenty of smoke and no magic; White London, a cruel city warped by magic and ruled by power-hungry dictators on marble thrones; Red London, where people and magic flourish together in harmony; and the legendary Black London, destroyed by its magic and closed off from the others.
Well, I didn’t know that much about the setting before. That sounds really interesting and neat.
Five more Londons after that, ending with The Bone Season by Samantha Shannona — here the setting is a London-yet-to-come, set after the year 2059 and straddling a boundary between fantasy and SF, with both supernatural and high-tech elements. Interesting!
All the choices on this list are interesting, though the only one I’m personally familar with is Neverwhere. There are probably — certainly — a lot more fantasy London settings Hewitt doesn’t mention. Let me see if I can add a handful more.
6) Un Lun Dun by China Mieville, a book I didn’t actually like and didn’t finish, though I can’t quite remember what the problem was.
7) Midnight Never Come by Marie Brennan. Fantastic title, and a detailed, beautifully drawn fantasy London.
8) Stretching a point, but DWJ’s Chrestomanci books are set, if not actually in a fantasy London, then definitely adjacent to a fantasy London. Chrestomanci’s own house is so separate from the city that I’m not sure it counts, but what the heck, it occurred to me, so I’ll include it here.
9. Jonathan Strange and Mr Norrell by Suzanna Clarke. Obviously wide-ranging, but a significant part of the very long novel is set in London.
10. The Anubis Gates by Tim Powers — a lot of that is set in London.