After reading, "Novichok agents: a historical, current, and toxicological perspective"[1] it looks like the only places to have produced such agents are Russian chemical weapon labs and the Organization for the Prohibition of Chemical Weapons Central Analytical Database.
Maybe someone who works in the field can provide some more evidence either way?
[1]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6039123/#R19
https://onlinelibrary.wiley.com/doi/abs/10.1002/rcm.7757
https://analyticalscience.wiley.com/do/10.1002/sepspec.1591c...
I am pretty sure that the Porton Down military research facility next to Salisbury is capable or producing it too - but that doesn't mean they produced the specific poison used to attack the Skipals.
The same for VX gas that was used to kill Kim Jong Un's brother in Malaysia. VX was indeed invented by the brits but I would seriously doubt that they killed Kim Jong-Nam.
Edit: this is just factual information. What kind of fscking loosers do downvote this? Go get a reality check.
Of course, you could argue that British intelligence framed those Russians during the investigation, and poisoned the Skripals themselves as some sort of ruse for expelling diplomats. But if you deny the evidence and proceed down that path, you've made the decision not to alter your beliefs and no discussion will change your mind.