Entering and remaining on premises is not in itself a criminal offence. It becomes an offence in a number of circumstances, primarily where a trespasser refuses to leave on being requested to do so by a displaced residential occupier.
a tenement taken in some unclaimed part, without purchase or permission
Squatting is the act of occupying an abandoned or unoccupied space or building that the squatter does not own, rent or otherwise have permission to use. The author Robert Neuwirth suggests that there are one billion squatters globally, or about one of every six people on the planet.