At least one aspect of illegality is that the invasion was with the specific purpose of regime change, although this was denied at the time. Iraq was no threat to any of the so-called coalition of the willing and was, at that time, the most closely overlooked country in the world. It was Bush and Blair who withdrew the weapons inspectors; it was not at the behest of Sadaam Hussein, although his degree of co-operation with them was fairly lukewarm. There were no grounds for believing that there were still WMDs present in the country and all "evidence" produced was completely spurious. Remember the dodgy dossiers, one of which was plagiarised by Alistair Campbell from outdated research that had previously been published on the internet. Does anyone else remember Colin Powell being made a complete fool when he demonstrated the mobile chemical weapons factory – something rather more like an ice cream van? Robin Cook, who had been Foreign Minister, seems to have been the only member of the government with a conscience.
The Chilcott Enquiry was pretty much an expensive whitewash. The UK and USA are both members of United Nations and no-one should be above the law.