Source, in full
No known direct citation to Gladstone; first attributed in early 1900s (e.g. Highways and byways in London, 1903, Emily Constance Baird Cook, Macmillan and Co.) but appears in late 1800s London guides by other authors, such as
Notes from Wikiquote
The best way to see London is by the omnibus lines. | A Tour Around the World in 1884: or Sketches of Travel in the Eastern and Western Hemispheres (1886) by John B. Gorman