Vil Mirzayanov, one of the inventors, of 'Novichok' seems to believe that Russia's authorities are behind the Salisbury attack. However, he also makes clear that a number of states would be capable of producing it.
He published in full the formula to produce 'Novichok' in a 2008 book. He says that given the technical complexities, only a few state actors may be able to produce it (among them, the UK, which may have used it to identify the toxic agent in Salisbury)

https://www.novayagazeta.ru/articles/2018/03/15/75815-nado-bylo-navsegda-lishit-rossiyu-ee-sekreta https://social.giorgiocomai.eu/attachment/22961