From what I understand, the incident took place in what is recognized internationally as Iraqi territory. Iran can claim whatever they please, but that does not mean their claim holds any weight outside of the Iranians minds.
The issue you speak of is nothing other than a diplomatic talking point aimed at stalling a response, but it wouldn't matter if they made that claim or not, the British won't respond with anything other than talk, or else their 15 sailors wouldn't be in Iranian custody.
It was an act of war, and the U.K. froze and backed down when they should have fought. Perhaps this will at least be a wakeup call for the rest of our forces in the region. If such an incident happens again, they should now be prepared to fight instead of wimping out as the British did on this occasion.
Jeez louise, wouldn't it have been better to defend them on the water than try to get them back from Iran? If they're not prepared and willing to defend themselves they should just go home.