My take so far:
Iran, besides the home front stuff, is doing two things.
First, it is seeking to drive a wedge between the UK/US alliance. The Iranian leadership knows as well as anyone that the British presence in Iraq is not popular in the UK. They may be hoping that taking the sailors and marines hostage will hasten and over all UK departure from Iraq - Shia Basra especially. Not having the British in Basra increases the Iranian ability to influence Shia Iraq, and further isolates the US both politically and tactically as it endangers their withdrawal route to the sea and Kuwait if things go really bad. This, combined with emerging developments in the Kurd - Turk - US equation, could create a real military problem for the US as they lose a safe exit - Syria and Iran are not options (Axis of Evil and all that).
And secondly, in light of US rhetoric and carrier deployments off the coast of Iran, having UK military hostages puts a dent in any designs the US has on attacking Iran as a unilateral US strike on Iran now would effectively end UK cooperation in Iraq. Cheney-Bush must be foaming at the bit. Blair must be shitting himself. Iran probably isn't as concerned about its international reputation as much as it is about controlling the real military threat on its borders. Iran can only assume the US means business at this point - carrier battle groups exercising off your coast would make anyone think so.
EXPANDED: All Iran really has to say to the UK is, "Leave Iraq and get your people back. Stay, and we'll keep them." As reprehensible as that is, it is definitely no more obscene than the unprovoked, ill-thought, invasion, occupation, and destruction of a nation. If the British can negotiate their release, obviously all the better, but because Iran has been threatened with war by the US and now sanctions* by the UNSC, it really has no incentive to let them go. If Blair has any sense left, he'd see the political writing on the wall in the US (not to mention the UK!), and withdraw anyway, humiliating as that may be for him. I fail to see what advantage remaining in Iraq gives them other than a [rapidly expiring] place at the table in Crawford, some nostalgia for empire, and an entry for Blair in a history book.
Isolating the US in Iraq is a bit like calling a bluff. Bush can bluster and harrummph about Iran all he likes, but when Iran starts stripping away what few allies he has, they are effectively narrowing the terms of engagement - mano y mano. Iran isn't caving, they're pushing back.
Without a shot being fired, through a small and simple action, Iran has given itself a significant political and strategic advantage.
*I tend to think sanctions often do little but increase tension in an already tense atmosphere. Looking at the history of states under sanction (North Korea, Cuba, Iraq pre-invasion, etc) they all seem to last and maintain the same leadership, whilst the average citizen suffers. But they make good "look, we're doing something" propaganda.