The US and Britain had both abandoned aggressive UN weapons inspections in the spring of 1998. UN weapons inspectors were able and willing to conduct intrusive no-notice inspections of any site inside Iraq, including those associated with the Iraqi president, if it furthered their mandate of disarmament. But the US viewed such inspections as useful only in so far as they either manufactured a crisis that produced justification for military intervention (as was the case with inspections in March and December 1998), or sustained the notion of continued Iraqi non-compliance so as to justify the continuation of economic sanctions. An inspection process that diluted arguments of Iraq's continued retention of WMD by failing to uncover any hard evidence that would sustain such allegations, or worse, sustain Iraq's contention that it had no such weaponry, was not in the interest of US policy objectives that sought regime change, and as such required the continuation of stringent economic sanctions linked to Iraq's disarmament obligation.
The British were never willing (or able) to confront meaningfully the American policy of abusing the legitimate inspection-based mandate of the UN inspectors. Instead, London sought to manage inspection-based confrontation by insisting that before any intrusive inspection could be carried out, it would have to be backed by high-quality intelligence. But even this position collapsed in the face of an American decision, made in April 1998, to stop supporting aggressive inspections altogether.
In the end, the British were left with the role of fabricating legitimacy for an American policy of terminating weapons inspections in Iraq, supplying dated intelligence of questionable veracity about a secret weapons cache being stored in the basement of a Ba'ath party headquarters in Baghdad, which was used to trigger an inspection the US hoped the Iraqis would balk at. When the Iraqis (as hoped) balked, the US ordered the inspectors out of Iraq, leading to the initiation of Operation Desert Fox, a 72-hour bombing campaign designed to ensure that Iraq would not allow the return of UN inspectors, effectively keeping UN sanctions "frozen" in place.
As of December 1998, both the US and Britain knew there was no "smoking gun" in Iraq that could prove that Saddam's government was retaining or reconstituting a WMD capability. Nothing transpired between that time and when the decision was made in 2002 to invade Iraq that fundamentally altered that basic picture.
But having decided on war using WMD as the justification, both the US and Great Britain began the process of fabricating a case after the fact. Lacking new intelligence data on Iraqi WMD, both nations resorted to either recycling old charges that had been disproved by UN inspectors in the past, or fabricating new charges that would not withstand even the most cursory of investigations.
The reintroduction of UN weapons inspectors into Iraq in November 2002 was counterproductive for those who were using WMD as an excuse for war. This was aptly demonstrated when, in the first weeks following their return to Iraq, the inspectors discredited almost all of the intelligence-based charges both the US and Britain had levelled against Iraq, while failing to uncover any evidence of the massive stockpile of WMD that Iraq had been accused of retaining.