British recognized the difficulties of the US position on Iraq in 2002

Ray McGovern has published an essay worth excerpting.

McGovern writes that: At that meeting [July 23, 2002], Foreign Secretary Jack Straw noted that the evidence regarding "weapons of mass destruction" was "thin."

McGovern quotes a July 21, 2002 briefing paper:
U.S. views of international law vary from that of the U.K. and the international community. Regime change per se is not a proper basis for military action under international law...Legal bases for an invasion of Iraq are in principle conceivable...but would be difficult to establish because of, for example, the tests of immediacy and proportionality.

A few more excerpts from McGovern's essay:

Grasping at straws, the document raises the possibility of demanding Iraqi acceptance of an unacceptably intrusive U.N. inspection regime:

It is just possible that an ultimatum could be cast in terms which Saddam would reject (because he is unwilling to accept unfettered access)....However, failing that (or an Iraqi attack) we would be most unlikely to achieve a legal base for military action by January 2003.

The British, you see, knew that the summer months in Iraq are uncomfortably hot. Thus, January was the time they thought an invasion would have to begin, or the attack would have to be put off until autumn. As for a possible attack by Iraq, British government documents released to Parliament show that American and British aircraft dropped no bombs on Iraq in March 2002, 10 tons of bombs in July, and 54.6 tons in September. Nevertheless, this failed to provoke Saddam Hussein into the kind of reaction that could be used as an ostensible casus belli. And intrusive inspections? Iraq wound up tolerating the strictest inspection regime in modern history. And when U.N. inspectors found Al Samoud missiles with a range greater than that permitted, Saddam allowed them to be destroyed.

One can visualize the British lawyers wringing their hands: Foiled again.
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To his credit, British Admiral Michael Boyce, chief of the defense staff, demanded a straightforward, written opinion from the attorney general that attacking Iraq would be lawful, before Boyce would put his troops at risk of subsequent prosecution as war criminals.

This put the bite on Attorney General Goldsmith who had long shared the doubts of the legal establishment about the legality of starting a war without unequivocal endorsement by the United Nations. After much equivocation, Goldsmith bowed to Blair and was asked to appear before the cabinet on March 17, 2003, two days before the war began. Goldsmith read a brief statement saying he now thought attacking Iraq was lawful, and Blair quickly moved the discussion on. Questions were not permitted. The British attorney general reportedly confided to lawyer friends during February and early March 2003 that he found himself in an "impossible" position, and wondered aloud if he should stay in the job.

Admiral Boyce, upset that he was never shown Goldsmith's more equivocal advice to Blair prior to March 17, has now said that if British troops are brought to trial by the International Criminal Court (ICC), British ministers should be "brought into the frame as well." The London Observer asked Boyce if Blair and Goldsmith should be included. "Too bloody right," was his answer.

American forces, of course, do not have to worry about the ICC, since the Bush administration "unsigned" the signature that President Bill Clinton had affixed to the treaty in December 2000. Nor have U.S. government officials shown themselves to be sticklers about international law. In November 2003, Richard Perle, then a key leader of the Defense Policy Board and a principal intellectual author of the invasion of Iraq, left international lawyers astonished when he told a London audience, "I think in this case international law stood in the way of doing the right thing."