In same way you can be arrested for "going equipped" or banned from near sports stadiums for football hooliganism, it is rarely held up in court but a tool for the police to preempt people who are positioned to commit a crime. There must be reasonable grounds otherwise one has a case for false imprisonment, I am guessing The Met took all reasonable steps to make sure they were targeting the right people using precious resources on what was one of the biggest police operations of the decade.