^^^ As I read it, Barry, it's not because Royal Mail won't deliver to addresses on the site but because the site owners won't permit it. That's almost certainly because they've only got planning permission for 'non-residential' units (which would explain why 6Robin0 has to vacate his temporary home for one month of the year).
If the local authority becomes aware that people on the site are having 'regular' mail delivered there, they could taking planning enforcement action against the site owners. However an occasional delivery from, say, Amazon is far less likely to be a cause for concern.
A large non-residential site not far from me, here in Suffolk, has recently been subject to enforcement action after the local authority became aware that many people on the site were living there permanently. The site owners where 6Robin0 is living are clearly trying to avoid similar problems.