Your captain's authority does not include the power of arrest ... but you can make a ctizen's arrest,
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.
As Captain of an aircraft or ship you too have certain extra powers, once the doors are shut. They do not exist with doors open though, as then the local civil power is deemed to be in charge.
Leastaways, that's my understanding.