Mostly it depends on the land owner. If you find the relevant guidebook for the area there will most likely be accurate information about where it is accepted that you can climb and where you can't. Some land owners object and crag access i banned, in others they are supportive. Sometimes landowners change and the next owner has a different idea and the access regime changhes. That said, there is a huge number of places to climb in the UK. Another good place to look is on the crags section in the logbooks here - very often has the ltest information. Find the crag you are talking about on the map and see what it says!