Not only in the UK .;-)

The problem is the population density. There are just to many people who want to enjoy the woods. Several options. You can just use dead wood and debries. (This is usually a bigger effort) Read the map and go to most inaccessible spots. Maybe a bush chair or similar small item is “fair use” of resources. Leave as little trace as possible (no tree trunks, no stone rings at fireplaces etc.) Or you can ask someone with private property.