Without prejudice to any existing right of the Crown, there shall be exempted from so much of the provisions of this Act as relates to buildings and structures, every building, structure, or work vested in, or in the occupation of, Her Majesty, her heirs and successors, either beneficially or as part of the hereditary revenues of the Crown, or in trust for the public service or for public services ; also any building, structure, or work vested in, or in the occupation of, any department of Her Majesty's Government for public purposes or for the public service.