The legality argument seems odd to me. We are going into another country, without the permission of the government of that country? Ah well, you can make it up as you go along.
I reckon that Cameron is feeling that it's in the bag, hence the 'terrorist sympathizer' gibe. He doesn't have to be polite now, although he will probably give a half-apology in the Commons. Well, he is OK, the Blairites will vote for war.