Oi Farmer, Get Off The King's Land!

We went for a New Years Day walk through the beautiful South Down's today and as you walk around the rolling hills and winding pathways you see notices nailed to walls and fences everywhere saying Private Property and generally telling you to keep out of this bit of the land else you can expect to be chased by an angry farmer telling you to get off his land and i wondered who decided what bit of the country is free for everyone to wander around and which parts are no-go zones?
Apparently it all goes back to 1066 and William the Conqueror who decided as he was now the English King, he should own absolutely everything in the country and declared himself absolute owner of the entire country and set about making the laws which stated that land cannot be owned by anyone except the Monarch.
As the chief landlord, he then began distributing vast swathes of it to his supporters giving them rights over the land, but not ownership but as they could hardly manage a county-sized chunk of land on their own, they would create lesser tenancies who paid him rent and these sub-tenants could then create their own sub-sub-tenancies, and so on so there was soon a whole chain of nested tenancies paying rent to each other, from the king at the top to the serfs toiling the fields at the bottom.
To answer my own question about the South Downs then, a bit of digging found that the vast majority (95.8%) of the 402,040 acres of land is owned by 3 Viscounts, 2 Dukes, a Baron, 2 knights and a couple of families and the remaining 4.2% (16,951 acres) owned by the National Trust after it was bequeathed to them on the death of the former owner.
Ultimately then, next time you enter a field and some angry inheritance dodging farmer runs across the field waving a stick at you and telling you to get off his land, you can politely explain to him that it isn't his land and unless he is King Charles, he can go stick his head in a cow pat.