Depends on how much jingoism runs in London at the time, really.
If the British decide to strike back, nobody will prevent it; but itll cause a huge shitstorm for little benefit. If cooler heads prevail, they can get concessions from Teheran because of their sudden casus belli. Actual strikes would be unwise, but not neccessarly unsuccessful.
Basically, Iran basically put itself in Serbia's shoes, circa 1914.
Not a bad simile in many ways.
Unfortunately things do not move at the relaxed pace of the summer of 1914. It was a full day before the London and Paris papers had the assassination story published. The satellite news organizations would have the entire history of the Iranian/West stand-off and the pictures of the location, the body bags and the politicians reacting inside of two hours, right into every home with a TV. It took five weeks for thing to go to hell then, mostly due to the time it took to mobilize. The U.S. and British can unload on Iran inside of an hour.
That is one of the worst features of today's crisis management. There is NO time to manage, no time to think. It just happens (or it happens TO you).