The ultimate test of the sovereignty of the new Iraqi government will be whether it has control over the occupying forces after the handover. Will it be able to direct coalition military policy (tightening the US rules of engagement so that they don't shoot so many innocent people, for example), veto politically sensitive operations (such as a repeat of the Fallujah massacre), or ultimately ask the coalition forces to leave and never come back? Britain says "yes". The United States says "no". Which one do you think is actually calling the shots?
If the US is only going to hand over "sovereignty-lite", then the rest of the Security Council should tell them where to go. The UN should not sully its hands with acting as a rubberstamp for US Imperialism. The Americans will no doubt claim that refusal would make the UN irrelevant, but I can think of no better way to make the UN irrelevant than for it to fatally compromise the ideals on which it was founded.