I have not seen sitting upright on a park bench making strange hand gestures looking up to the sky among the typical symptoms consistently associated with nerve gas poisoning in any of the literature I have read.
It's usually more like convulsions, involuntary loss of bowel and bladder, foaming at the mouth, lacrimation, lying on the ground struggling to breathe and actively dying a violent and horrible death, etc.
May is specifically accusing Russia of using Novichok which is reported to be up to 10x more potent than VX. I would think the accepted and consistent symptoms of nerve agent poisoning would be unmistakable if something that potent were used.
IMO, this sounds more like carfentanil which the Russians have been known to use from time time to time. It's a narcotic that is about 100x more potent than fentanyl which is itself about 80x more potent than heroin and there is concern about its use as a WMD due to the extremely small doses required to induce respiratory arrest. It is also able to absorb into the blood stream via the transdermal route afaik.
Also interesting that washington post is now describing noth victims as being found "unconscious on a bench" despite the initial eyewitness accounts p[reviously referenced that suggest they were conscious.
I'm not an expert on poisoning but the British government seems really sure on this. I suspect they have another intelligence source which they are not discussing - the guy was working for MI6 for quite a few years after all.
"the most toxic nerve agent known to man"?