Diehard, it the Royal Navy was within it's right to seize the weapons and boat because there were reasonable grounds to suspect it's use was for piracy. The RN is not required to prove intent was for use of piracy, only to have reasonable grounds to suspect that is the case. Where are you getting this info that somli militia are equiped with motherboats & skiffs, machine guns, rpg's, extra fuel and grappling hooks. That is what has bene released by the media, we don't know what other evidence was to suggest they were pirates. The pirates weren't arrested because the RN could not link them to a specific incident.
You even quoted an british official stating - "As a British official put it: “We can only arrest suspected pirates if we catch them in the act or on the point of launching an attack on a vessel. Clearly, with all the weaponry in the skiffs, there was an intent to commit piracy, but we hadn’t actually caught them in the middle of an attack so we had to release them.”
Un security council resolution 1851 states-
“2. Calls upon States, regional and international organizations that have the capacity to do so, to take part actively in the fight against piracy and armed robbery at sea off the coast of Somalia, in particular, consistent with this resolution, resolution 1846 (2008), and international law, by deploying naval vessels and military aircraft and through seizure and disposition of boats, vessels, arms and other related equipment used in the commission of piracy and armed robbery at sea off the coast of Somalia, or for which there are reasonable grounds for suspecting such use;
Solid, irrefutable proof is not required.