'Correct me if I'm wrong.'
You're wrong.
The British public are very angry about this, but prefer to follow the advice of Winston Churchill on matters of this nature, I quote:
"Jaw, jaw, not war, war."
In this situation we, like our US cousins always explore diplomatic solutions before military ones, especially when we are already thinly spread, militarily, because of Iraq and Afghanistan. When a third front is opened in Iran, it will be because the US and Britain choose to, not because Iran provokes them into it.
We all hope and plan for the best possible situation. If history is to be a judge however, typically an aggressor does not give a defender much of a choice. Japan attacked the US. Germany attacked Europe. Initiative in war has tremendous advantages. Most aggressors utilize those advantages. The fact that the US and Britain are stretched thin with weak domestic support, helps to make us an easier target. We should expect the Iranian attack when we are at our weakest point.
A statement of sanity not heard often these day's.
Thank You
"....but prefer to follow the advice of Winston Churchill"
When it comes to third fronts, Churchill's advice often left much to be desired..eg The Dardonelles.