The Met has, to date, arrested around a thousand suspects now, out of 1,500 arrested UK wide.
To make an arrest at the scene of an incident takes at least two officers, who then are required to spend a couple hours processing the suspect. That means the police would need twice as many officers on duty as they had. Not practical in the real world.
Also to make an arrest after the event, usually means the evidence required to secure a conviction is more likely to be in place, it is considerably less confrontational and means less bystanders become involved.
