Ah, things are becoming a bit clearer. I was right to suspect that the charges against the British soldiers would be incursion into Iranian waters, especially considering that disputes over the border between Iranian and Iraqi waters in the Shatt-al-Arab was the genesis of the Iran-Iraq war.
If the Brits really were seizing vessels in Iranian waters, or using Iranian waters to do so, then all of a sudden the Casus Belli has reversed itself- the Brits have (arguably) committed an act of war against the Iranians, instead of the other way around.
(That is, of course, assuming that the invasion of an Iranian consulate by American forces a while back wasn't already. The U.S. had a figleaf there, claiming that it wasn't really an embassy so it didn't count, but I somehow doubt they'd accept that argument were it the other way around.)
So, the question being raised by the right wingers (if not the progressives; where the hell ARE they on this issue?) is "why now?" That's actually a good question, but not quite for the reason you'd think, assuming it's not just a coincidence.
According to the linked story, today's a holiday in Iran; that's why it's been tough to get in touch with anybody in Iran today. Now, that might mean that the Iranians timed this deliberately so that they wouldn't have to diplomatically respond to the inevitable firestorm of criticism.
Another possibility that I haven't seen discussed, though, is that it's the Brits taking advantage of this. If they provoke a response today, then they get the opportunity to get out in front on this issue- to condition the world media's response, and set the idea into the public's head that they were operating in Iraqi waters, not Iranian. By the time the Iranians respond, the UK (and, of course, the United States) firmly control the perception of who is in the wrong and who is in the right on this. By Monday, Iran is an international pariah.
Either is possible. The latter is somewhat Machiavellian, but that doesn't make it impossible, just a little less likely.
More as this develops. After all, somebody's gotta write on this who isn't baying for Persian blood.