There is more than one way to get into the POW status, although most of us are used to ( http://www.genevaconventions.org/ )GC III, Article 4, for standard conflicts, there is also Article 5: "Art. 5. The present Convention shall apply to the persons referred to in Article 4 from the time they fall into the power of the enemy and until their final release and repatriation.

Should any doubt arise as to whether persons, having committed a belligerent act and having fallen into the hands of the enemy, belong to any of the categories enumerated in Article 4, such persons shall enjoy the protection of the present Convention until such time as their status has been determined by a competent tribunal."

Now, as Iran is alleging that these are spies, although they were in uniform so no doubt should be around, they obviously have some problems with the status of the Brits. Therefore the full benefit of Article 5 is in force, so that they are provided the full POW benefits until they have a tribunal, and are to have 2-3 weeks to prepare for said tribunal and to have consul that is competent.

Iran is claiming belligerent act in the form of espionage and these individuals in uniform are to be provided full POW coverage until such time as their status can be determined. If Iran had asked via civil authority means for the ship to move out of its waters, it would have shown understanding and leeway to pure commercial shipping. It did not, claims espionage which is a belligerent act and so now does need to act in a civilized way towards those that have a status in doubt.

That is just my reading of the GC, others may differ.