I'm afraid both you and British Admiralty Law are outdated with regard to international law. Under customary international law, pirates were considered "hostis humani generis" or "the enemy of mankind" and any country could arrest and try them under their jurisdiction. The modern international law governing piracy is the 1958 Convention on the High Seas, and its successor, the 1982 U.N. Convention on the Law of the Sea (UNCLOS). UNCLOS defines piracy as:
(a) any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed:
(i) on the high seas, against another ship or aircraft, or against persons or property on board such ship or aircraft;
(ii) against a ship, aircraft, persons or property in a place outside the jurisdiction of any State;
(b) any act of voluntary participation in the operation of a ship or of an aircraft with knowledge of facts making it a pirate ship or aircraft;
(c) any act of inciting or of intentionally facilitating an act described in subparagraph (a) or (b).
The same acts, when committed inside the territorial waters of a country, do not fall under the definition of piracy, but are simply considered "sea robbery" under international law, and are dealt with by the laws of that country. Domestic laws seldom permit a vessel or warship from another country to intervene. Illegal acts committed for political rather than private ends also fall outside the international law definition of piracy.
Under UNCLOS, all signatory countries are required to cooperate to the fullest possible extent in the repression of piracy on the high seas. Any country may seize a pirate ship or a ship taken by pirates, arrest the pirates, and that country's courts are entitled to decide on the penalties to be imposed. However, UNCLOS does not apply to sea robbery taking place within the territorial waters of a country, meaning that any rights and obligations it imposes are useless outside of the high seas. Most piracy this year, particularly that taking place off the coast of Somalia, has occurred inside territorial waters and is not covered by UNCLOS. This has left the victims of Somali piracy in a perilous situation, with no grounds to intervene under international law, and given the political climate in Somalia, no enforceable domestic laws to assist them.
To legally intervene against a ship on grounds of piracy the ship in question must have committed an act of piracy. Cruising around with a boatload of guns and rockets is not illegal, nor is it an act of piracy under international law - no matter how suspicious. What HMS Portland did was an illegal seizure and destruction of a privately owned Somali boat in addition to the illegal confiscation of cargo (weapons).
Disregarding the kindergarten antics this has been an interesting thread, but I think it has run its course.