One of Saddam's stated reasons for initiating the Iran-Iraq war was that Iran was denying him access to the Gulf by claiming all of the Shatt-al Arab waterway. The cease fire agreement placed the border down the middle of the main channel, allowing both sides access. Shortly before the beginning of hostilities under Operation Iraqi Freedom, Saddam Hussein deeded the entire waterway back to Iran's sole control. I think Britain is, legally and only on paper, in the wrong here. There are principles such as adverse possession that may have bearing here, if Iran has never before attempted to enforce control since Saddam gave it back. I also do not recall whether Iran officially accepted Saddam's offer, or legally took possession after the gift was offered, as there was some implied quid pro quo support inherent with Saddam's gift. This was about the same time as when Saddam released Kuwaiti POWs and made other concessions in attempts to rally support against the Coalition.
Bottom line, Britian will probably negotiate unless Iran tries to milk this too much.
In the broad view, I believe that Iran suspects hostile intentions from a second term Bush administration and is attempting to force the issue prior to the election. In any event, siezing the British boats hasn't lenghtened the term in power expectancies of any of the Ayatollahs, Mullahs or officials.
I think this whole episode is a non-issue, and will be resolved shortly. I've thought that since it was reported early this morning. But I do think the Iranians may have sabotaged Iraq's pipeline. And it doesn't mean that Iran can't have clandestine activity going on that they'd like other's to keep their noses out of.