SW10 West Brompton: why this London postcode has the number 10

Last in the alphabet, after Stockwell

West Brompton is SW10 because of the alphabet. SW1 is the South Western district office, and in the Post Office's list of March 1917 the offices after it follow by name: West Brompton comes after Stockwell (SW9), and is the last of them.

See SW10 on the map