Yeah. It's opportunism on the part of the Iranian President. I doubt anyone set out to capture British sailors, but they were close enough to, or maybe even in, Iranian waters and someone got over-eager to defend their borders. The problem is that if they try to use these guys (and girl) as a bartering chip, we set a precedent whereby our soldiers are no longer just in fear of being blown up by roadside bombs, they have to fear (more than before) being kidnapped too.
I can see why people believe diplomacy should be used here - but the longer we wait for the return of the prisoners, the higher the chance they are mistreated (and I don't need to point out that Iran has a poor record on human rights, especially towards women). It's dangerous enough to invoke military action (post-rescue things could be talked down) - but if our people are harmed it takes this to a whole new level. A level we aren't ready for yet.