This, the Hayling Island Heavy Anti-Aircraft Gun Site, is not marked on Dronescene for any reason (apart from my recommendation) although it is a site where soldiers lost their lives.
It now appears to be under the care of the local council. I wondered whether the military had an residual interest in your pillbox.
As part of the stop line it may have been associated with a nearby operational base for the Home Guard Auxilliaries - the secret volunteers who would go to ground as the country was overrun and emerge to carry out acts of sabotage and terrorism on the occupying forces.