One of the big questions about any upcoming American attack on Iran is the justification they'd use for it. They need a casus belli.
They now have it.
My guess is that there's more going on here than we're reading in the news. There is nothing in the Guardian or in, from what I've seen, any other papers about why Iran did what they did. Certainly the Iranian commanders aren't dumb enough to think that there wouldn't be repercussions for this activity, so there would have to be a justification from the Iranian side.
(Judging by how touchy they are about it, I'd imagine that the British were either acting on the Iranian side of the Shatt-al-Arab river, or interdicting ships headed to Iraq from Iran, or both. The Royal Navy apparently insists they were in Iraqi waters. They'd probably do that no matter what, not being blithering idiots themselves, so I don't find that entirely credible.)
(I know that if I were looking to set up a casus belli by deliberately provoking the Iranians, this would probably be how I'd do it.)
In any case, for those out there panting at the thought of getting to attack Iran, might I remind you that you already have two wars that you haven't won yet? I'd suggest dealing with them before opening up a third front, especially when one of those (Iraq) will become a nightmarish insurgent hell the second the bombs drop on Tehran.