...it's not recognized at all.
Nope, it's not. Not officially.
However:
http://www.sealandgov.org/historySince Roy was still a British citizen a summons was issued under the fire arms act and on the 25th of November 1968. Roy and Michael were in the dock of the Crown court of Chelmsford assizes in Essex. There was much argument and laws going back to the 17th century were called upon. During his summing up the judge said This is a swash buckling incident perhaps more akin to the time of Sir Francis Drake but it is my judgment is that the UK courts have no jurisdiction. This was Sealands first de facto recognition.