My! That's a contradiction if ever I saw one!
Joao, for your info of course you have the power of arrest as long as we're talking UK law and UK juristiction, Captain's "power" doesn't enter into it. How the hell could citizens arrest exit without it? In fact you have exactly the same powers of arrest as any police officer, and they have no more or less than you do. They just have more training in the ins and outs of the business, and certain extra powers, but arrest is the same. They are citizens too, remember; servants thereof, as they sometimes need reminding.
There is a big difference between the power vested in, say a constable, compared to a citizen.
Succinctly, the citizen must have reasonable grounds to believe that an indictable offence is being or has been committed, whereas a constable only has to have a reasonable suspicion.
If you perform a citizens arrest and an indictable offence has not been committed, you could find yourself in all sorts of trouble, as well as being subject to the person you have 'arrested', using reasonable force against you in self defence.
All I was saying is that an aircraft captain cannot arrest someone on suspicion, as they do not have this power.
Pilot YGC, Lord Spandex Master is an airline captain and he says he would negotiate a meeting with a constable off the aircraft; in my opinion this is a reasonable and wise view. However, if the aircraft is not in flight, it is non sequitur that the captain can overrule local authorities.
When the aircraft is in flight, it is another matter completely and the captain has very high levels of authority, including choosing to break other laws to ensure the safety of aircraft and occupants.
This whole diversion came from Diplome making an incorrect assertion, perhaps it is now time to return to the theme?