From the longer Wikipedia page [1]
The Cenotaph is a war memorial on Whitehall in London, England. Its origin is in a temporary structure erected for a peace parade following the end of the First World War, and after an outpouring of national sentiment it was replaced in 1920 by a permanent structure and designated the United Kingdom's official national war memorial.
Designed by Edwin Lutyens, the permanent structure was built from Portland stone between 1919 and 1920 by Holland, Hannen & Cubitts, replacing Lutyens' earlier wood-and-plaster cenotaph in the same location. An annual Service of Remembrance is held at the site on Remembrance Sunday, the closest Sunday to 11 November (Armistice Day) each year. Lutyens' cenotaph design has been reproduced elsewhere in the UK and in other countries of historical British allegiance including Australia, Canada, New Zealand, Bermuda and Hong Kong.