Deek writes:

They [the photos] are from Bushy Park on the outskirts of London.

During WW2 Bushy Park on the Surrey outskirts of London was the site of Camp Griffiss which was the transit, training and admin centre for the USAAF (United States Army Air Force) in the UK after their entry into the European theatre of war in 1942, and later was the HQ of the USSAFE (United States Strategic Air Force in Europe). This marker is one of sixteen that record the bounds of four blocks of temporary offices and accommodation within the park.
The camp was named after Lt. Col. Townsend Griffiss. He was the first American serviceman to be killed in the European theatre. Unfortunately it was a 'friendly fire' incident when the aeroplane in which Griffiss was travelling was shot down by the RAF.

Nearby is a further memorial in the form of a plain, oblong, bricked surface with an inscription tablet. This is the site of General Dwight D Eisenhower’s SHAEF office (Supreme Headquarters Allied Expeditionary Force), where Operation Overlord was planned in 1944.

Deek