It is also likely that people outside the Russian government have can produce or have access to Soviet stockpiles of Novichok. In addition, it appears that Novichok refers to a several different chemicals, and the exact compositions are classified. Therefore there is no way to independently check the UK’s identification of Novichok.
For example alternate sources, here is a link to a New York Times article from 1999 about efforts to recover tons of Novichok from Soviet stockpiles left in the former Soviet Republic of Uzbekistan:
There are also reports that a Russian chemical weapons expert was attempting to sell the technology in Syria. There is speculation that the Mossad had him killed:
This has not been a banner year for the SVR (I’m guessing the wet work department they called SMERSH during WW2). First the Ukrainians faked the murder of that journalist to roll up their whole network in country and now this.