Invading Syria would have been a disaster. For starters, as even Cameron admits, there is no definite evidence that Assad used chemical weapons against his own citizens, so we aren't even certain who the 'enemy' would be if we invaded.
Aside from the glaring problem that we don't know who we're fighting, we also have no right to just barge into Syria and open fire on whoever we like. Britain is in no way entitled to judge other countries and interfere as it pleases. I'm pretty sure that the Iraqis and Afghans would agree - whenever Britain gets involved in foreign wars, it only ever ends in tears.
Tears, and a hell of a lot of taxpayer's money. Even if Britain were really able to help in Syria (questionable at best), we're only just crawling out of the recession, and expensive and drawn-out foreign wars should not be our priority right now. More money needs to be spent on services like the NHS, education and creating jobs in order to boost our flagging economy instead of on wars abroad that will only worsen our economic problems.
"Money needs to be spent on the NHS, education and creating jobs instead of on foreign wars"
Perhaps we've finally learnt something from our time in Iraq and Afghanistan - that entering foreign wars in order to show our international prowess benefits nobody, least of all the people in the countries we set out to help.