Burden of proofs is on accuser. Period.
I doubt proof of anything matters much to Russians.
Rationale, law, decency, protocol are not harboured by the minds of this frame-up - the same minds who will do anything to conduct a Nuclear First strike against Russia as per their own documents. Demonization campaigns must be created or events massaged beforehand to produce sufficient percieved cause for this action. It will be interesting to see what's fabricated next in the smear campaign. Perhaps they'll hire the screenwriters from the film "Wag the Dog" to craft another story.
But, it seems Porton Down's own scientists can not link Russia to the source of this supposed poison.
And, being accused of something it had nothing to do with, naturally Russia wants some questions answered by the UK and France. Will they answer?
The guy really is insane
So is this Finnish guy: