18(1)Persons may be detained under paragraph 16 above in such places as the Secretary of State may direct (when not detained in accordance with paragraph 16 on board a ship or aircraft).
(2)Where a person is detained under paragraph 16, any immigration officer, constable or prison officer, or any other person authorised by the Secretary of State, may take all such steps as may be reasonably necessary for photographing, measuring or otherwise identifying him.
(3)Any person detained under paragraph 16 may be taken in the custody of a constable, or of any person acting under the authority of an immigration officer, to and from any place where his attendance is required for the purpose of ascertaining his citizenship or nationality or of making arrangements for his admission to a country or territory other than the United Kingdom, or where he is required to be for any other purpose connected with the operation of this Act.
(4)A person shall be deemed to be in legal custody at any time when he is detained under paragraph 16 or is being removed in pursuance of sub-paragraph (3) above.