I have just received an email from the chair of Borough Green Parish Council
You have stumbled on one of our little Borough Green secret idiosyncrasies !!
The sign was put up by a young man from the village with the help of some college friends as a prank. He was then tragically killed in a mountaineering accident in Scotland. The sign was spotted by Dot and Dave Sutton, and much hilarity ensued with press speculation, followed by some correspondence with KCC Highways, who saw both the joke and the tragedy, so it was decided to leave the sign as it was as a small memorial to a lad that cheered us all up, unless someone complained it was "unauthorised". But no-one ever has.