The war was not illegal, no further UN mandate was required.
Absolute nonsense. The war was illegal. A UN Security Council resolution was required. If a second resolution was not required, why was it sought? The UN are not approached as a "courtesy"; the UN is the abitrer of its own authority and the Secretary General said the war was illegal, after it became apparent that the causus belli - WMD and supposed non-compliance with an inspection regime - was entirely fabricated.
If this premise is used, then there are many UN resolutions for which non-compliance could be cited as authority for war. At the time of the Iraq war, there were almost 90 UN Security Council resolutions being violated by nations other than Iraq. Israel and Turkey were frequent violators, and how desirable would it be to witness an attack on Israel or on Northern Cyprus citing breached resolutions?
The UK attorey-general's advice on the legality of military action was equivocal until he was told to come back with the correct answer. The only reason that no legal action is likely to be brought against the architects of this war is that they are still in power.
Any prospect of military action against Iran should be viewed from the outset as illegal. Nothing has changed since 2002 - no lessons have been learned or mistakes admitted.
Incidentally, I do not agree with Kendall-Smith's argument as there was a UN resolution in place at the time of his refusal, and he was not in a position in which his actions may have been contrary to laws regarding the conduct of war.