Porton Down is very close to the spot where the attack took place so they might be responsible, there are no evidence neither proving or disproving any theory at this point.
Conspiracy theory, no evidence given.
Many competing and contradictory stories promulgated by Russian state-controlled media, the only connecting thread being to confuse their audience and exclude any possibility of Russian involvement. https://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 Russia 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, https://theconversation.com/what-we-know-about-novichok-the-newby-nerve-agents-linked-to-russia-93264
In its statement, the European Union expressed shock at the offensive use of any military-grade nerve agent, of a type developed by Russia, for the first time on European soil in over 70 years. http://www.consilium.europa.eu/en/press/press-releases/2018/03/19/statement-by-the-foreign-affairs-council-on-the-salisbury-attack/