1881 – London Population 4.7 Million when Holmes and Watson take 221b
1896 – London Population 6.7 Million during The Adventure of the Bruce-Partington Plans, wherein secret submarine blueprints are found in a murdered engineer’s pocket.
1920 – London Population is 7.4 Million when Holmes is 66 and The Dangerous Drugs Act is enacted… Sherlock Holmes and the Remaining Improbable takes place.
In 1860, when the six year old Holmes was still home in Yorkshire (about 322 km and or 200 miles away) London was already one quarter larger than the world’s second largest city, Beijing; two thirds larger than the Paris; five times as big as NYC. By 1881, Holmes had moved to London. Rents were high. The population was young, a little more female than male. He was on the lookout for a fellow to share the rent.
The Criminal Investigation Department had recently been created, naming 250 detectives, including Lestrade, in a police department that numbered 10,000.