You should take the time to read this in its entirety. This takes the timeline back as early as April 2002 and shows that that the UK was only following Bush's lead...
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http://www.timesonline.co.uk/article/0,,2089-1648758,00.html10. Aside from the existence of a viable military plan we consider the following conditions necessary for military action and UK participation: justification/legal base; an international coalition; a quiescent Israel/Palestine; a positive risk/benefit assessment; and the preparation of domestic opinion.
Justification
11. US views of international law vary from that of the UK and the international community. Regime change per se is not a proper basis for military action under international law. But regime change could result from action that is otherwise lawful. We would regard the use of force against Iraq, or any other state, as lawful if exercised in the right of individual or collective self-defence, if carried out to avert an overwhelming humanitarian catastrophe, or authorised by the UN Security Council. A detailed consideration of the legal issues, prepared earlier this year, is at Annex A. The legal position would depend on the precise circumstances at the time. Legal bases for an invasion of Iraq are in principle conceivable in both the first two instances but would be difficult to establish because of, for example, the tests of immediacy and proportionality. Further legal advice would be needed on this point.