The Brits may not be as spineless as they appear at first blush with regards to their own personal "Iran Hostage Crisis", which the U.S. so meekly endured almost 30 years ago. I initially set out to pen a "why doesn't England and the U.S. take decisive action here - members of the military have been kidnapped by a foreign government, for crying out loud!" post. Perusal of lots of Guardian, Jerusalem Post, Bloomberg, etc. articles ensued, and as I got fired up about each one, I was continually struck by the odd lack of urgency or saber-rattling on the part of Great Britain. Now, it all makes sense.
We begin with two mortal enemies, Iran and America. I would say that Iran currently is, without a doubt, our most vocal and antagonistic enemy while considering their perceived ability to actually stand up to us. At the same time, seizing our sailors (if it were even possible) would amount to signed authorization on their behalf allowing us to obliterate their country. Not advisable. However, take the same action against Ruth ("wherever you go, I will surely follow"), er, Great Britain, and sit back and watch the fireworks (threatened, that is; no ACTUAL conflict would ensue, by Tehran's calculation)! With somewhere north of 20% of the world's oil flowing the Strait of Hormuz and an Iranian navy whose express purpose is to give the appearance of the ability to completely disrupt that flow, I believe Iran staged this ruse NOT to delay UN sanctions related to their nuclear program, but rather to cause an immediate-term oil shock and demonstrate/send a reminder of their un-attackable status to the world (while adding a few dollars to the oil coffers in the process). Nothing more. And it accomplished that very objective, if only for a matter of hours, with the price of oil spiking $5 almost instantaneously on the news of the capturing. But by the next morning, oil had settled down (no doubt after a few well-placed phone calls from the leaders of Britain and America).
Now, Iran is playing and looking the part of the idiot, with nothing to show for their effort other than some well-documented Geneva Convention violations by using prisoners as propaganda and the realization that they don't have nearly as much control over the price of oil as they believed they did. The ability to disrupt the flow through the Strait is only half the equation; the other half is having an actual reason to do so, and without that, oil prices don't care how many Iranian ships float on those waters, since to harm the flow of oil without provocation would harm Iran as much or more so than anyone else. Britain has shown and continues to show (calculated) restraint, and their avoidance of the knee-jerk reaction has harmed Iran's leaders on the international stage moreso than any aerial or naval strike possibly could have.