bjcc
You use the word detain again. There is no such power, other than for the purposes of a stop and search. This was obviously not a stop and search. If it were, the following principles apply.
1 Principles governing stop and search
1.1 Powers to stop and search must be used fairly, responsibly, with respect for people being searched and without unlawful discrimination.
1.2 The intrusion on the liberty of the person stopped or searched must be brief and detention for the purposes of a search must take place at or near the location of the stop.
1.3 If these fundamental principles are not observed the use of powers to stop and search may be drawn into question. Failure to use the powers in the proper manner reduces their effectiveness. Stop and search can play an important role in the detection and prevention of crime, and using the powers fairly makes them more effective.
1.4 The primary purpose of stop and search powers is to enable officers to allay or confirm suspicions about individuals without exercising their power of arrest. Officers may be required to justify the use or authorisation of such powers, in relation both to individual searches and the overall pattern of their activity in this regard, to their supervisory officers or in court. Any misuse of the powers is likely to be harmful to policing and lead to mistrust of the police. Officers must also be able to explain their actions to the member of the public searched. The misuse of these powers can lead to disciplinary action.
And each person detained would have to be informed of
(a) the constable's name and the name of the police station to which he is attached;
(b) the object of the proposed search;
(c) the constable's grounds for proposing to make it
And if there were a power to "detain" other than for the purposes of a brief search, do you seriously suggest that the person detained would not be entitled to an explanation as to why he was being detained, and food and drink and lavatory facilities from the person who was detaining him?
Most importantly, after each person had been detained and searched, he would be free be go about his lawful business. The power cannot be used to detain hundreds of people at a time.