Iran wanted to humiliate the British by sanctimoniously proclaiming there would be no negotiation for marine/sailor release until the Brits admitted their cohort was in Iranian waters.

The British have responded with the smoking gun proof that indeed it was Iran that violated Iraqi waters and in an act of war kidnapped the British Marines/Sailors.

Obviously Iran is using some kind of threat on the captured Brits, for the lone female was made to say on video the Brits were in Iranian waters. So despite the pictures of the captured Brits heartily eating a meal, some kind of mind manipulation or extortion is in play.