If Nalvany was poisoned this time it was almost certainly Russia, but I could also just see him dying. He'd already been poisoned, had complained about being denied outside food (and possibly wasn't eating a lot of what they gave him for fear of being poisoned), had complained about the conditions etc. Not a combination that's very conducive to good health.

As for whether MI6 or someone else could have done this sort of thing in the past and had the capability... also almost certainly.

I remembered this Channel 4 News fact check from after the Skripal poisonings here. The whole thing is an interesting read but main relevant points in broad terms are:

• Basically any country with a chemical weapons program (including 'defensive' like everyone claims - in reality there is no difference) could make Novichok and similar agents usually linked to Russia. All it takes is time, effort, and the right lab. In fact, any country would want to do so at the very least to classify it, add it to a database, and work on potential protections.
• One of its original chemists released information on its compound and processes publicly in the 90s. Russia eventually jailed him, but there's no way foreign intelligence didn't already have more than he published.
• In the mid-90s the US involved itself in a 'clean up' operation at a site in Uzbekistan where various strains of Novichok were allegedly made, so probably had direct access to the work.

That being said, I think the Salisbury poisonings were probably Russia too given the evidence that came out over time. Not that another explanation is impossible.

Also, the history of Porton Down and the UK's involvement in various off-book chemical weapon related stuff is long and extremely dark and shady - from Anthrax attacks, to the black market, to our probable involvement in the Apartheid regime's genocidal chemical warfare programme Project Coast.