Since the authorities are not talking about their plans, it would be a guess, but they are probably thinking days rather than weeks. PM Blair mentioned days as being the appropriate time unit for resolution of the situation.
But Blair (UK) isn't the one holding the cards here (in regard to this reaching the point of no return). The Iranians are......Which is why I can see this going another week or so before reaching that point.....Where they can't just spit out some hyperbole, declare a quasi victory and release these 15.....(at that point the UK will do nothing).
However, if this turns into a real legit, longterm hostage situation....with the Iranian notion of a full elaborate trial and punishment......Then?
Well then do the people of the UK have the stones to cowboy up and do whatever it takes to rectify this situation....Blair has the stones to, his warriors do.... but I'm not so sure if he has the support within the UK populate as a whole for it.....
Meanwhile Iran reaps the windfall profits for oil prices...