Russia is accusing the United Kingdom of preparing a chemical attack under a false flag at Idleb, whilst the United States is accusing Syria of exactly the same thing. The virulent exchanges that have taken place within the UN Security Council may give the impression that a Western intervention is imminent.
In actual fact, the UK only has a basis for accusing Syria if the latter deploys its land forces at Idleb. This is because chemical weapons cannot be dispersed by air but only using land projectiles. Till now, the only preparation for a land attack on Idleb is in the form of Russian air bombings targeting al Qaeda establishments. Nothing more.
If the attack on Idleb were to start before 6 November 2018, the date of the elections of the US House of Representatives and a third of the Senate, Syria would be exposed to a fresh attack from the West. A Syrian Government attack on Idleb would be basis enough for the United Kingdom to carry out its operation under a false flag to force President Trump to intervene with urgency, that is, without having the time to verify the allegations made by the United Kingdom. If he failed to do this, Donald Trump would take the risk of losing the mid-term elections and finding himself charged (impeached) by the new Congress.
Therefore a US attack would be an extremely lethal act of war, contrary to the previous acts.
It therefore makes more sense for Syria to wait for these elections before liberating Idleb.