Sch 2 para 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). (1A) But the detention of an unaccompanied child under paragraph 16(2) is subject to paragraph 18B. (2) Where a person is detained or liable to be 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. (2A) The power conferred by sub-paragraph (2) includes power to take biometric information (within the meaning given by section 15 of the UK Borders Act 2007) . (2B) Paragraph 4(7) to (9) applies to sub-paragraph (2) as it applies to paragraph 4(5). (3) Any person detained under paragraph 16 may be taken in the custody of a constable, an immigration officer, or 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.