Last resort? To me, the most interesting revelation from the fight was that (a) Zatara could have communicated with his daughter at any point (assuming you take that at face value, which I do), and (b) Paul apparently never even tried to get Wonder Woman or anyone else to put pressure on Nabu to allow communication with Zatara.
Let's not pretend that Paul's fig leaf about arresting Nabu was serious. Nabu's best response actually would have been to just call Paul's bluff, and offer to go with him to the nearest police station, whereupon I'm rather certain that the response from the London police would be that they didn't want anything to do with this.
Paul never honestly tried to get the authorities on his side and have Nabu arrested, nor the lawful path of simply filing a lawsuit against him. This was always going to be about violence and force.
I'm fairly dubious about Siskin being a legitimate agent of the police here. Were the legalities to matter, I would rather expect that if the City of London were asked whether Paul and his cohort were actually acting as official agents of the London police, the official response would be "Fuck no!"
In any event, at the point your plan is to mind-rape someone with fear, any reflexive response you provoke will almost certainly be lacking the necessary mental state for a crime to be committed.
And, if you're going to pretend that Paul's actions were simply legitimate law enforcement (and that a magical entity is a person), the obvious next question would be at what point Nabu was actually incapacitated and should have been turned over to the London police, as opposed to whatever is left of him being given to John Constantine.
Under U.S./British common law, you had the right to use deadly force to resist
an unlawful arrest, downgrading any death to (at most) manslaughter, if you meet the standard for that. Most states have eliminated that right by statute; no idea what the current British law is.
