Wellington Arch, also known as Constitution Arch or (originally) the Green Park Arch, is a triumphal arch located to the south of Hyde Park in central London and at the north western corner of Green Park (although it is now isolated on a traffic island). The arch, and Marble Arch to the north of Hyde Park, were both planned in 1825 by George IV to commemorate Britain’s victories in the Napoleonic Wars. The Wellington Arch was also conceived as an outer gateway to Constitution Hill and therefore a grand entrance into central London from the west. The presence of a turnpike gate at this point had led, in the 18th century, to a strong perception that this was the beginning of London (reflected in the nickname for Apsley House as “No 1, London”) and the arch was intended to reflect the importance of the position.
Yes that is copy-pasted straight from Wikipedia. Why? Because up until two minutes ago I had no idea what this monument was. We walked by (under) and when I saw the joggers coming through the arch on the other side I ran ahead to get this shot.