US/UK assassination policy

A recent news article on the reorganisation of British military forces stated that Britain still wouldn’t sanction assassinations of foreign leaders. Thinking about Saddam Hussein in particular just what is it that separates an assassination from a war exactly in legal terms. Apparantly an attempt was made on SH’s life during the Gulf War when a bombing raid was made on one of his bunkers with the intention of taking him out. Is there anything that let the US lead coalition attack Iraq but couldn’t let say, a sniper take out SH alone.