Hi Helo
I thought Parkhurst was a restricted area, however according to ANO it isn't. I suspect the map might be more up to date than your GPS.
Just as a matter of interest, when I was last flying in that part of the world (twelve years ago, possibly soon after an escape attempt by helo at another prison) I was told that the concern was not that a helo was coming to rescue an inmate but that the lags would think one of their mates was about to be liberated and leap on the nearest prison officer. Don't know if there was any truth in that but maybe that is behind the thinking!!
Cheers
TeeS