Apart from the tens of military advisors and senior commanders in the UK and US who advised against it logistically (or at least advised that way more troops and a way longer time were needed) and were ignored...
Here's a good article.
I think it's safe to say that you could substitute Blair for Bush through most of it too, especially given that for some reason Bush's govt decided to use British advisors on Arabism (and even then way after they should've appointed any advisors: i.e. before the invasion!).
And yep, I know it's from the socialist scoundrels that are the Guardian, but if you're gonna argue about it at least try refuting some of the points - I dare ya!