To some extent you are arguing against yourself.
Blair should be brought to book because he lied to the people and waged a war on the international scene which has been deemed illegal by the world of nations.
Even then, if the war has been deemed illegal by every other country in the world, if it was in the vital interest of UK national security and the wellbeing of the British people, the UK has the power and the ability to simply say "tough it was right for the UK, if you don't want a war don't threaten us". That I would support unreservedly.
However it was not in the interest of the UK national security or the wellbeing of the British people.
No, it was not in the interests of UK security or our wellbeing to go to war. We were not under threat. Saddam's Iraq could not hurt us. Apart from a few conventional Scuds it could only just hit Israel or Riyadh or Dammam. There were no WMDs, no nuclear, it had no air force to speak of, there was a no-fly zone.
I sat in Al Khobar when Saddam's Scuds came in. They were wayward and the Patriot missiles took most of them out. One got through, though, and killed 27 Americans less than a mile away form where I was based. Saddam's Iraq was not, and never was, a threat to the UK, yet Blair made out it was and sent UK military to their deaths. I will never forgive him for that.