Porton Down is mired in controversy, among other things growing cannabis and harbouring aliens.
Summary
Skripal and his daughter may have overdosed on Fentanyl.
Disproof
Conspiracy theory, no evidence given.
Many competing and contradictory stories promulgated by Russian state-controlled media. euvsdisinfo.eu/disinformation-cases/?text=sergei+skripal&disinfo_issue=&date=,
The toxin was identified as a nerve agent at Porton Down, which is Britain's military research base. They have not done tests with nerve agents since 1989, but are still able to identify these toxins, in order to protect British civilians and troops from attacks http://www.bbc.com/news/av/uk-43335148/porton-down-britain-s-secret-research-facility. The nerve agent 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
British Prime Minister Theresa May said Monday a “military-grade nerve agent” known to have been developed by Russia was used to poison former Russian spy Sergei Skripal and that her government has concluded it is “highly likely” Moscow was responsible.
“Either this was a direct act by the Russian state against our country or the Russian government lost control of this catastrophically damaging nerve agent and allowed it to get into the hands of others,” she said.
Further debunking by Julia Davis twitter.com/JuliaDavisNews/status/971924556423356416, .