Kevin Drum asks: "If Britain believed that Saddam Hussein's regime had no significant ties to al-Qaeda, why did Tony Blair support war against Iraq?" He claims the answer is that Britain truly believed Iraq had WMDs. I don't think that's quite right, although eventually they leaned on that rationale. Listed in the "Options Paper" is the main reason why, I think, the British government preferred regime change over containment and deterrence:
Within our objectives of preserving peace and stability in the Gulf and ensuring energy security, our current objectives toward Iraq are: the reintegration of a law-abiding Iraq which does not possess WMD or threaten its neighbors, into the international community. Implicitly, this cannot occur with Saddam in power.
Now it's hard to say exactly why the British government placed such an emphasis on "the reintegration of a law-abiding Iraq into the international community," but that seemed to be an overriding concern here, and WMDs were only one part of it. Now the interesting twist is that the British government also didn't think "regime change" was a viable military objective. Here's a memo written by Peter Rickets to the Prime Minister:
Military operations need clear and compelling military objectives. For Kosovo it was: Serbs out, Kosovars back, peace-keepers in. For Afghanistan, destroying the Taleban and al Qaida military capability. For Iraq "regime change" does not stack up. It sounds like a grudge between Bush and Saddam. Much better, as you have suggested, to make the objective ending the threat to the international community from Iraqi WMD before Saddam uses it or gives it to terrorists. This is at once easier to justify in terms of international law but also more demanding. As with the fight against UBL, Bush would do well to de-personalize the objective, focus on elimination of WMD, and show that he is serious about UN Inspectors as the first choice means of achieving that
So basically, the British government thought the main rationale for war was to preserve peace and stability in the Gulfand preserve "energy security"by reintegrating Iraq into the international community, something that couldn't be done with Saddam Hussein or even, necessarily, another Sunni General in power. But the British also seemed to realize that the only way to achieve this, "nation-building," was too vague an end state to make for a viable military objective. And thus they hoped to straddle this contradiction by focusing almost entirely on the WMD rationale and hoping that Saddam Hussein would refuse to let inspectors into Iraq (a prediction that was discussed in the "Options Paper"). But as it turned out, Saddam did let inspectors in, which screwed up that rationale. But by that point both the United States and Britain decided to blunder into war anyway. What emerges through all of this is just how muddled the planning was; not only was the rationales for war cocked up, but none of the people in charge seemed to be sure, exactly, what they even hoped to achieve from war.