Prisoners

See section 13 of the Prison Act 1952.

In section 4 of the Official Secrets Act 1989

Official Secrets Act 1989

The Official Secrets Act 1989 is an Act of the Parliament of the United Kingdom. It repeals and replaces section 2 of the Official Secrets Act 1911. It is said to have removed the public interest defence created by that section....

, the expression "legal custody" includes detention in pursuance of any enactment or any instrument made under an enactment.

Children

See section 86 of the Children Act 1975.

Mental patients

Any person required or authorised by or by virtue of the Mental Health Act 1983

Mental Health Act 1983

The Mental Health Act 1983 is an Act of the Parliament of the United Kingdom which applies to people in England and Wales. It covers the reception, care and treatment of mentally disordered persons, the management of their property and other related matters...

to be conveyed to any place or to be kept in custody or detained in a place of safety or at any place to which he is taken under section 42(6) of that Act is, while being so conveyed, detained or kept, as the case may be, deemed to be in legal custody. For this purpose "convey" includes any other expression denoting removal from one place to another.