Many of you are forgetting, that if Britain still sent forces to Iraq, especially after the Commons voted NO, two things are likely to happen.
The first is that a vote of No Confidence would be held against the Government. Now considering the Commons voted NO against the war, but the Government sent troops anyway, you can bet your bottom Pound that the Commons would find No Confidence in the government. As a result, the government would have to resign to make way for someone else.
Secondly, that someone else, although may not last long as a government, would last long enough to call the troops back.
And this then underlines the second fundamental issue in this situation - that being the Crown can only act on the advice of the Government. It's Constitutional Law. The Royal Perogative can only thus take place if the PM advices the Crown to do something (indeed almost everything) - let alone send troops to war.
Of course that's if it got that far. Blair, I'd extremely doubt, wouldn't go against a vote in the Commons, because he'd know the next vote in the Commons, if he ignored the No to war vote, would be the No Confidence motion. And he'd know he'd lose that &, needless to say, government.