Maple - UNSCR678 covered the removal of Saddam from Kuwait. The subsequent resolution covered his conduct and reparations for the invasion.
It's very convienient to use 678 as a justification for war but in reality that resolution was specific in it's aim and didn't cover anything outside of it's remit.
As for 1 million people marching in London against the war - well people at the time thought that the democratic process was being ignored and that Blair was on a route to war no matter what. Strangely enough in a lot of ways they were right - the WMD's as was didn't have a 45 minute time to readiness-they didn't threaten Cyprus -Iraq wsn't an operating base for Al Qaida(it is now!) and various other reasons wich have now pretty much been debunked.
So in essence - stick to the Mod pathlets on why we went to war if you wish - however many think that the oil was the reason and a bit of family revenge !