The British government is allying old, imperial patterns of thought with American physical power in order to enforce our intentions on Iraq.
In, say, 20 years' time, there may be some quite different power centres in the world: maybe a Chinese one, or a vast Islamic confederation, or a pan-African empire. Then, one of these powers may be in a position to act in the name of the United Nations and to inflict its own policies on Britain. We would deeply resent that. We would want UN representatives to bend over backwards to see our point of view and to try and build channels of trust with us.
It is not enough for the UN to use the empire mentality of any individual members as a vehicle to carry out UN intentions. A new thought process is needed, and Britain and the US are not contributing towards this.