Look, it is now clear to me.

The British must have known where they were to within yards if not feet with modern satnav.

The commander of the Cornwall must have known both where the border is and where the Iranians say it is.

Therefore either the British were playing for this situation (which does seem rather improbable) or the Iranians were lying more than usual inspired by religion (indeed any religion) which often has that effect.

We've all taken harder decisions than that.