Interfering in the internal politics of another country, let alone joining one side in a civil war is a dangerous thing to do. In the case of Syria, there is not a united anti-government side in the way that there was in Libya. Even so, partisan support in Libya was not supposed to be the plan, just protection of civilians. Any sort of action in Syria would involve ground forces – are we, is the Government prepared to commit British soldiers in this way? Is it legal?
Regime change is very dodgy. I'm sure we all agree that Assad is a nasty piece of work, but if Assad, then why not North Korea's beloved successor, Robert Mugabe, Vadimir Putin, etc. etc.? And what happens if another country decides regime change is necessary in a country that is one of our allies?
The lack of action on the part of NATO and the British Government has nothing to do with those people who, correctly, protested against the Iraq War. No country in the world was under such intense oversight as Iraq before it was invaded in 2003. To smuggle out any chemical or biological weapons would have required an invisibility cloak. There was absolutely no reason to believe that any viable chemical or biological weapons still existed in Iraq at that time. And now look at the state of the country…