It was stored in several of the Republics, to include Ukraine.
Britain has its own stockpile of Novichok, conveniently being stored at a military bio-weapons facility a few km from the crime scene.
maybe some of those little green men stayed behind in Ukraine?
Why not do the same with the nerve agent as was done with the Syrian nerve agent - send it to Brussels. Russia, if they like, can send their own sample to Brussels of the named agent, and the labs there can compare the two, and with the results from Syria. Throw it at the walls of the UN. That way it’s not one or two countries accusing Russia and it would be hard for Russia to challenge any results Brussels came up with.