It won't necessarily get the servicemen back but there are more important considerations in the long run that the lives of 15 soldiers.
The interesting one now is that they have become caught up in internal Iranian politics, in a rather large way. The latest accounts seem to be suggesting that the commander of the Revolutionary Guards wants to give them back as fast as he can get rid of them, while his 2IC wants to put them on trial as spies. I think there is potentially quite a lot of benefit here for us to light a very slow fire under the pot and stew the lot of them, but we probably want to stop short of any shooting as if that happens they're more likely to pull together. My best guess right now is that the establishment faction wants to get rid of them, and the Ahmedinajad faction wants to put them on trial and make a spectacle out of them (and I suspect this was the plan all along - the seizure was way too rehearsed, and they were moved to Tehran far too fast for anything else to happen). If so, their plans have already gone somewhat awry.

Sometimes I really believe you uk people have lost the ruthlessness needed to deal with crazies with your destruction as their goal. You spend too much time thinking about looking bad.
I think it's a case of we're not capable of it, but are only willing to do it in extremis. Gas warfare is a good example - despite thinking that we had superiority over the Germans in it, the one time it was authorised was against a German beachhead had Sealion gone ahead. Short of the survival of the UK, we weren't willing to stoop to such levels.
Right now, virtually nobody in the UK thinks that the Islamist nutters are actually capable of destroying or even seriously damaging us. They're generally perceived to be a nuisance, and are not thought to have nation-state backing. This in turn means we aren't willing to use very much force in dealing with them, percieving them to be a legal rather than military issue by and large.

Personally, I've found it is much more effective being feared than loved when dealing with psychopaths.
And were the world composed only of psychos I would agree with you. The problem is that the majority of states are very rational, and one states view of you often spills over into another. Having the Iranians viewing us as psychos would unfortunately lead to other (friendly) states doing the same, and thus diminishing our influence with these states.
All that is necessary for the triumph of New Labour is for good men to say nothing whilst CGS.