It looks to be an old pump of some kind but why would this be in the middle of an ancient woodland?
A little research proved that in 1946 Buckinghamshire County Council built a camp for Polish soldiers and their families from the Third Carpathian division in Italy as they could not safely return to Poland after the Cold War when Poland was passed from German to Russian hands.
The area was full of temporary buildings, barracks buildings and Nissen huts and was home to 156 Polish families. The camp was eventually demolished in 1961, but is still remembered with a commemorative plaque.