5.1 NECESSITY FOR AN ARREST WITHOUT WARRANT: CONSTABLES - S.24(5) PACE 1984
(1) It is believed the arrest is necessary:
(a) to enable the name of the person in question to be ascertained (in the case where the constable does not know, and cannot readily ascertain, 'the persons name or has reasonable grounds for doubting wether a name given by the person as his name is his real name);
(b) correspondingly as regards the persons address
(c) to prevent the person in question -
i. causing physical injury to himself or any other person;
ii. suffering physical injury