Generally I find that the BBC doesn't exactly lie, it just uses a lot of emotive language and puts undue emphasis on some things and ignores others, in a way that has nothing to do with being objective and neutral.
Of course you shouldn't take the bbc at face value, thanks to labour after iraq etc it has become somewhat legally a government mouthpiece, and still prominent labour supporters are shouting from the rooftops because some journalists and reporters are conservatives, so think about the creep when Labour next take hold.
But that doesn't mean they are impartial and why we do have substantial soft power across the globe, they are not on the levels of RT yet.
As for documented proof, the UN have documented proof of around 28 attacks from the Assad regime using chemical weapons, and with Russia being on the UN and SC boards nothing much gets done.
Considering this air strike was nothing more than a PR stunt and Russia/Syria have been notified of targets so troops can get out, this is not an "evil west" scenario.
If you are trying to insinuate a false flag you have to put on your tin foil hat and exclude the previous instances of documented proof and give a credible reason as to why we have let Russia back Assad since 2013.