Enjoyed this thoroughly. ("When a man is tired of London, he is tired of life"--Samuel Johnson).

My own favorite hidden treasure (from the brief times I was there): the Leighton House.

Speaking of hidden, the 19th century writer Hubert Crackanthorpe wrote about London's lost rivers, and the group Coil set his words to music: http://transpont.blogspot.com/2014/01/music-monday-coil-lost-rivers-of-london.html

As a travel writer, you would probably really enjoy Crackanthorpe's slender travel memoir, "Vignettes," from which the above lines were taken.