The issue is the land is owned by someone. It's private land, there may be permissive routes but fundamentally, by setting up your tent you'd be trespassing. Do I agree? No, not at all, do I obey? Yeah, because the consequences of not following that law unfortunately cost quite a bit. I don't believe you get jail time, but the fines are pretty shite. There is a movement to bring in the "right to roam" similar to Scotland but of course you'd need to have protections in place for the owners of the land, farmers etc to protect them if the law of trespass were to be abolished. Litter, waste, damage etc are all massive problems to those land owners. They'd need legal protection from incurring losses due to other people using their land. It's a surprisingly complex arguement that's been ongoing for ages. But I am hoping we get the right to roam eventually.