Putin is thought to be testing how isolated Britain is after Brexit and he won't like the results

[Denzil_DC]

4. I think that claim and this OP are running away beyond currently known facts.

Here's another theory about what happened, for instance: https://www.democraticunderground.com/100210365868

It's no more a conspiracy theory than what Chatham House are currently suggesting above without hard evidence.

Among doubters, there have been claims that there's no way to trace the origin of the Novichok. My understanding is that this isn't clear, because the isotope ratios of certain elements in it could be used to identify their geographical origin.

That in itself wouldn't clear up the mode of delivery (there have been a few official stories about that so far - doorknob, luggage etc.), culprits nor motivation, but it would be a step forward.