Iran trying to split US and Britain?
Iran faces three fairly determined foes in the Middle East: Israel, the UK, and America. Any opportunity Iran has of sowing discord between them is sure to be seized upon.
The capture of the 15 Royal Navy personnel is just such an opportunity. The BBC offers an analysis of why Iran pulled its most recent hostage taking stunt. One small part of the analysis caught my eye: Iranian hardliners are arguing that the 15 should be traded for the five Iranian agents the US recently captured in Iraq.
By demanding a trade, Iran immediately puts pressure on both Britain and the US. Although this won't change the nations' policies toward Iran, it will cause political strains, all at little cost to Iran. Later, when military action is required, those political strains may turn into small ruptures.
This chance to throw Eris' apple of discord certainly explains Iran's withdrawing its promise to release its female captive. They must expect to do some hard bargaining in the near future. Freeing the woman once the bargaining process becomes difficult allows them to earn goodwill (at least theoretically).