The mainstream political opinion got us into Iraq, Vietnam, sold arms to the Indonesians in East-Timor, and concocted a nuclear missile gap with Russia that never existed. I don't think it deserves any credence except where concrete evidence is provided (and this goes for everyone else outside the mainstream too).
If one were to ignore political biases, I think they would admit that there was more motive for the rebels to stage a gas attack to create international pressure for a cease-fire that allows them to escape a surrounded and hopeless position, than for Assad to bring the wrath of the world down upon himself to gas a completely surrounded and hopeless enemy. If you don't at least admit that such a thing warrants investigation, you are not being honest.
Were you troubled at how quickly the "political mainstream" came to consensus on what happened there, before any investigation had occurred?
As a UK citizen, the implication that Britain was involved in poisoning its own citizens is not unlike how many Americans feel about '9/11 was a false flag'.