It's not about grounds to arrest them, it's about practicality.

If you need to arrest someone in London (someone who might be about to cause harm) then there's a very good chance they will go through a tube station, and they tend to have relatively few, easily controlled exits. Most tube stations have two or less.

A few high profile tube stations occasionally have armed police presence.

But I think the idea that face recognition is being used to suppress protest is pretty overcooked; apart from the utter silliness around the banning of Palestine Action, we have a solid right to protest and we do.

The main problem is London has far too few police officers, and it has a transport system that is under consistent terror threat.