Moscow wants to see if the government will be able to muster a strong response to assassination attempts on British soil.
https://www.chathamhouse.org/expert/comment/skripal-attack-test-uk
Even when feeding the swans on the banks of the River Avon or enjoying the peace of Salisbury Cathedral, Vladimir Pasechnik was always afraid.Long before Sergei Skripal was poisoned, another Russian enemy of the state lived and died, invisibly, in Wiltshire. His name has almost faded into obscurity
https://www.thetimes.co.uk/article/city-s-other-spy-lived-in-fear-of-kgb-revenge-fhkwhnvbp
http://www.ng.ru/courier/2018-03-12/9_7187_equilibria.html
https://www.radiosvoboda.org/a/29090475.html
https://www.radiosvoboda.org/a/29093918.html
https://newtimes.ru/articles/detail/152433
https://newtimes.ru/articles/detail/152286
http://www.rosbalt.ru/world/2018/03/12/1687989.html
https://www.inopressa.ru/article/12Mar2018/lemonde/magnitsky1.html