The legality or illegality of the war is a moot point- no country is going to test it in case it sets a precedent that backfires against them.
The same with the vote in parliament- nobody is seriously going to vote on the matter unless there is absolutely foolproof evidence that he lied in case it limits their own party when they are in power.
The press and minor parties can bang on about it as much as they want; as with Gladstone and the Nile Campaign it might even bring down the government; but at the end of the day the Conservatives and Labour (and the Lib-Dems if they thought they will ever get into power) will not vote for it because it threatens to tie their hands.