If the BBC is correct, then IMHO it was probably a deliberate provocation - "we'll fly as low and slow as we like, and wherever we choose, and we dare you to try to do anything about it."
Those birds usually float around at about FL600 probably out of reach of anything the Iranians have, and at that height they can afford to stay well on the Omani side of the strait and get nowhere near 'International Waters'