And now I know.

Apparently this is a Quintain. It's on the green of the village of Ofham in Kent. A Roman exercise was for a youth on horseback to run at it as fast as possible and hit it. Whoever managed to break the board was given the title of chief of the day's sport. I have driven past this almost every day for the last three years. It was only when my nine year old asked what it was that I stopped to have a proper look.