Yeah, because agencies and especially of good old colonial powers like UK and Dutch have historically been very trustable, and not at all working to promote the interests of their states (with lies, covert action, false flag operations, and so on)
>It implies that another actor would have tried to kill former Russian agents and UK citizens with a chemical weapon.
Yeah, nobody does false flags ever.
A more plausible explanation is that Putin waited 12 years to kill the guy, long after he was out of Russia (where they could have gotten to him far more easily), and from 2000 other ways to kill someone they chose something that's the equivalent of living their business card on the murder scene...