s 5 Provisional arrest
(1) A constable, a customs officer or a service policeman may arrest a person without a warrant if he has reasonable grounds for believingβ (a) that a Part 1 warrant has been or will be issued in respect of the person by an authority of a category 1 territory, and (b) that the authority has the function of issuing arrest warrants in the category 1 territory. (2) A constable or a customs officer may arrest a person under subsection (1) in any part of the United Kingdom. (3) A service policeman may arrest a person under subsection (1) only if the person is subject to service law or is a civilian subject to service discipline. (4) If a service policeman has power to arrest a person under subsection (1) he may exercise the power anywhere.