"We may never know what actually happened. The area where the sailors were captured is at the mouth of the Shatt al-Arab, a border estuary that has caused two wars between Iran and Iraq since the 1970s. Iraq claims ownership of the entire estuary, while Iran wants it divided between the two neighbors. It is possible that the sailors thought they were in Iraqi waters while the Islamic Revolutionary Guard's Marines believed them to be on the Iranian side.
What is certain, however, is that the sailors would not have been captured without Tehran's approval at the highest level. The Brits based in Basra have often strayed into waters that Iran regards as its own; Iranians have also often passed into Iraqi waters.
In other words, these are almost daily incidents. The standard procedure is to warn the trespassers and guide them back to their own side of the water. If that procedure was abandoned this time, the reason must be someone's desire to provoke an incident.
If trespassing were the cause of the incident, one wonders why the Islamic Republic turns a blind eye to American vessels often straying into its so-called continental-shelf territorial waters. A casual boat ride in the Persian Gulf would offer the visitor countless examples of this on a daily basis..."
America, you see, has a Navy. And an Army. Airforce, etc. The poor socialist Brits have none of the above, so all they can do is come ask us for help, and this saves Iran from an immediate, on the spot, katie bar the door reprisal. Modern Brits enjoy diplomacy, too. Not that one can do all that much besides diplomacy when one is without an armed force, mind you, so it's talk, then more talk, while Iran gets to strut a muscle flex for moslems the world over to admire.
Picking on the Brits is about as low as picking on Togo.