If I were looking for a place to disappear, and was thinking I would do so without gaining a land owner's permission, I would be looking at places like this:
55.004479, -1.788896
Comes with other problems, access, noise, not a lot of room to move around, but unlikely to get regular foot traffic, no dog walkers, no shooting, no farmers or forestry workers.
Joking aside, Toddy's is good advice, but it would help if you shared a little more about what you are looking for. For instance, I know some folk who have made arrangements with land owners to teach bushcraft on their land, and have been able to set up semi permanent structures and leave some kit there. Your opening post could be read that way, or that you are looking for a place to build the kind of underground den/bunker/cabin that some folk show on Youtube. I think that if this is what you want, then why you want it might play into whether you get permission. A landowner might be okay with someone they have got to know building a permanent camp that they use once or twice a month for recreation, but might be less keen on someone setting up a camp that they want to occupy on a semi permanent basis, or who is planning to use it for a refuge in an end of days scenario.
All the best
Chris