ORAC,
This highlights the point that satisfying one of these three requirements is not a high hurdle to cross: Former Yugoslavia was covered by a slew of UN Security Council resolutions from 1991 onwards, (e.g. IFOR covered by UNSCR 1031 of 1995
http://www.un.org/Docs/scres/1995/scres95.htm; Kosovo, UNSCR 1244 of 10 Jun 99,
http://www.un.org/Docs/scres/1999/sc99.htm). East Timor by UNSCR 1262 (27 Aug 99) and 1264 (15 Sep 99) amongst others.)
Sierra Leone was a case of coming to the aid of a sovereign government, and in any event would have been legal to stop the crimes against humanity being perpetrated by the RUF.
As Kosovo shows, if the Security Council is deadlocked, humanitarian intervention was a legal - though controversial - basis for intervention. Happy to provide more detail if reqd.
As a result, UK operations since 1990 (GW1, Former Yugoslavia and Sierra Leone) have been clearly legal - except GW2.
S41