The real similarity between the case of Sergei Skripal and Alexander Litvinenko is the cynical way that the British authorities are exploiting it for anti-Russian propaganda.
It seems highly significant that Russia’s presidential elections are due to take place later this month. What better way to smear the expected electoral victory of incumbent president Vladimir Putin than to accuse the Kremlin of carrying out an assassination plot on British soil against a former Russian spy?
Alexander Litvinenko, was poisoned with radioactive polonium 210 in 2006. An inquiry into the murder, led by High Court judge Sir Robert Owen, found, in 2016, that Litvinenko was murdered by a former Russian agent, Andrey Lugovoi, with the “strong probability” that the murder was ordered by Russian intelligence.
The nerve agent that poisoned Skripal has been identified as Novichok (which means newcomer in Russian). It is known to be more powerful than VX and was developed in the Soviet Union in the 1970s and 1980s https://www.voanews.com/a/british-prime-minister-russia-poisoning-spy/4294683.html. It's so unusual, that very few scientists outside of Russia have any real experience in dealing with it and no country outside of Russia is known to have developed the substance https://edition.cnn.com/2018/03/13/europe/what-is-novichok-nerve-agent-intl/index.html