They know who did it - two colonels and a general in the GRU. I suppose you can say it was circumstantial in that nobody caught them actually putting the poison on their doorknob, but it's pretty well accepted that 3 officers from Russian intelligence travelled to Salisbury using false identification on the day of the poisoning and then left the country. Unless you believe their explanation that they were there to "see the cathedral."
Sergei Skripal was a colonel in the GRU who was also a spy for MI6 and who moved to the UK as part of a prisoner swap. Apparently he hadn't quite retired and was still briefing western intelligence services just before his poisoning.
> Apparently he hadn't quite retired and was still briefing western intelligence services
He had stopped been a Russian agent in 2004. Then, after being released from prison and moving to the UK, he had 8 years to brief the UK services about everything he knew. What kind of briefing could he do, exactly?
Note that if you read carefully the article that you linked, it says repeatedly that none of Skripal's activity was new, or uncommon, or important. Despite the subheader saying "may offer motive for poisoning" the various experts mentioned in the article repeatedly say that those "neither explained nor justified the nerve agent attack", "dismissed them as a likely motive for the poisoning attack".
https://www.bbc.co.uk/news/uk-48801205