Tony Blair (who still had a fondness for international law before Iraq was invaded) was always careful not to use regime change as a legal justification, because he knew international law didn't recognise it as a reason. The UN Security Council can authorize an attack to stop genocide - but that has to be explicit. Other than that, the reason has to be, I think, restoring
international peace and security, as quoted above. That means there has to be a problem with other nations. And the Security Council has to authorize it. Unilateral action needs to be true self-defence - none of this "he might have threatened someone some time in the future" crap.
Which puts Bush and Blair, but especially Blair, in a nasty spot. He ended up saying the original 1991 UN resolutions allowed the attack, and 1441 just continued that authorization. That claim is disputed by international lawyers. For instance ,see this piece written just before he gave that justification:
http://news.bbc.co.uk/2/hi/uk_news/2826331.stm
"There is a school of thought that going to war without the express authority of the Security Council would violate the UN charter," says Professor Grief.
...
Professor Anthony Aust was until recently one of the top legal advisers at the Foreign Office. He says previous resolutions allow the United States and Britain to use military action to restore peace and security in the Gulf region.
...
rofessor Warbrick agrees. Not only is there no imminent threat from Iraq, he says, but the UK Government has not demonstrated a link between the regime in Baghdad and any terrorist group who might pose such a threat.
Without a second resolution, he says, it would be unlawful for the UK to use military force against Iraq.
With the evidence for non-compliance with the 1991 Iraq disarmament resolutions now torn to shreds by the US and UK reports, and fairly obviously sexed up by the governments anyway, because they knew it was too weak in reality, there's a real case they invaded illegally.
That means little to Bush - I think the chances of any future American government prosecuting him are small (what president would want to set such a precedent?), and he's such a narrow-minded git that he'll quite happily not step outside the USA for the rest of his life.
But Britain signed up to the ICC. Even if the UK doesn't prosecute him, the rest of the world can try - and under the Extradition Act 2003, "For the first time, under the EAW, other countries will not be able to refuse to surrender a fugitive simply because there are one of their own nationals" - applying to any European Union countries. So if , say, a zealous Spanish judge issued an arrest warrant, Britain is meant to hand him over. And the Spanish police might even be able to come to Britain and do the arrest themselves - I'm not sure about that last part.
At the very least, Tony's going to have to think again about those vacations in Italy and the south of France.