If here is proof of their guilty, why then there is no court decision?
For me British version of case is just political speculation.
Skripal was arrested in 2004, court decision issued in 2006. Only in 2010 he was exchanged. Russia had pretty much time to make his "hearth attack" in the prison - 6 years.
After exchange he lived in UK 8 years. There was no reason for Russia to wait 8 years.
My version: Skripal was "discharged" by MI5 or MI6 after he told all what he knew and became useless, while UK was in need "to pay" to Russia for extermination of Litvinenko by polonium right in the center of London, after which UK secret services lost face. British laboratory which works with Novichok is located just in 15 miles from Salisbury. It was good opportunity to discharge useless double agent and to make troubles to Russians, and MI5 or MI6 used it. Unfortunately for them Skripal survived.
P.S. Sutyagin, who was swapped together with Skripal in 2010, still living in UK and even working for UK and NATO.
P.P.S. In early October 2018, British journalist Mark Urban published a book, The Skripal File, which, based on several interviews with Skripal himself, indicated that Skripal supported Russia's annexation of Crimea and Putin's policy towards Ukraine, and did not believe that Russia had poisoned him.
https://www.panmacmillan.com/authors/mark-urban/the-skripal-files/9781529006926