If you move in you are a tresspasser and the real owner can get a court order to evict you. If you live in the property for 12 years and no one tries to evict you then:
A if the title to the property is registered you can apply to the Land Registry to become the owner of the property. They tell the registered owner, and that person is then allowed to bring proceedings to evict you. If they do not the property becomes yours:
B if the title is unregistered no one can get an eviction order against you after 12 years, and you can aply to the Land Registry for Possessory Title.