So, certainly no surprise that some parts of CastleMartin show strava use
Exactly this. Most of Salisbury Plain is also open to the public at various times. A friend learnt to drive on the roads there at weekends, and another (an OTC Member on exercise) had a pizza delivered to a 6-figure grid reference by a local pizza shop who I suspect make quite regular trips to bored squaddies who don't fancy their ration packs that evening.
Equally however, there are times when you do not pass the closed gates and red flags if you have any regard for your immediate welfare.