I am cynical about these things, but I believe Cameron has been lucky. He has shown willingness to do what the Pentagon requires, but he now has a first rate excuse for not sending British forces into what could be a long and costly war. This will save the country a fortune in defence costs and human lives. It will probably also increase Cameron's chances of winning the next election. The worst aspect of it from his point of view, is that it will cause some temporary pain to his buddies in the armaments industry.
I won't make predictions, but I am hoping that this vote might have some knock on effect in the USA itself and help to avert US forces getting involved too.
Assad is undoubtedly an arsehole and a tyrant. However, the reason our Conservatives (and those elsewhere) wish him to be deposed quickly is that they perceive him as a left-wing tyrant and arsehole. The policy of supporting anyone who wishes to fight him appeals to those who have forgotten what happened in Afghanistan.