You and I might believe that proof, but we aren't the audience. Those who believe in bringing the US to heel won't drop that faith in the UN and will find reason to disbelieve that proof. They'll say that it was manufactured. Count on it.
I doubt that, even if the Administration provided solid proof, those others demanding a UN blessing would believe the evidence was credible (at least publicly). They'll howl that we need UN approval anyway and are acting as rogue nation. That's one probably reason Germany (Saddam's biggest supplier) now sits on the Security Council because they stand to give the howlers credibility and to deprive us of a unanimous vote. The propaganda would be useful to our enemies. I think most in the UN are making an attempt to entrap us with this game and would be delighted if Saddam succeeds in delivering a WMD to the US. It would help destroy the aura of invincibility and provide our enemies with both hope and more recruits. It would also put doubt into our protected "allies." Depriving us of UN approval would help strain our capabilities.
No, I think this is a very delicate situation and don't want to see the military get overextended. I also think that Powell has been a loose cannon. How are you so certain the administration is not giving the UN the rope to hang themselves, setting them up?
How are you certain that anybody outside the US and Britain cares?