Arrest? For what?
That's the point. A lot of these people make a living out of this, and will have on hand a number of solicitors who similarly make a living out of them.
If they stand outside a police station filming and taking pictures of everyone that leaves, they're not doing that for any reason other that to try and provoke (a) an encounter which they can film, edit and then put on social media for views, or (b) provoke an arrest / use of force which they will then sue on, and 99% likely the local police authority will simply pay out on. If a person behaves like a terrorist, but you are pretty certain they aren't - then you don't have power to search or arrest.