The reason that Britain isn't aggressively pursuing this yet is that it is not clear whose waters they were in.
"The seizure of the British vessels, a pair of rigid inflatable boats known as RIBs, took place in long-disputed waters just outside of the mouth of the Shatt Al-Arab waterway that divides Iraq from Iran. A 1975 treaty recognized the middle of the waterway as the border. Saddam Hussein canceled the treaty five years later and invaded Iran, triggering an eight-year war. Iran disputes Iraq's jurisdiction over the waters near the mouth of the Shatt Al-Arab."
While this may provide the pretext needed to go into Iran, Britain probably wants to wait and see how the US will back them. Without congressional approval Bush may be reluctant to help.
Times sure have changed since the War of Jenkins Ear. And for the record, who cares what Iran's position is?