.. and in order to do so, you must of course have access to the materials first of all, in order to create the spectrometric signature upon which your comparison - and thus, evidence - is made.

So, no. You can't claim its only available in Russia, if you've positively identified it using these signatures .. since you'd have had to have the material to compare with, yourself ..

I didn't claim that only Russia was able to make the chemical agent.

I don't entertain the false flag narrative because it is so difficult to disprove. If the UK was willing to kill this man and endanger the lives of their citizens to destabilize Russian relations, then pretty much anything is possible. Even if this was some special nerve agent that only Russia is able to make, the UK could simply kill the man with a similar nerve agent and claim it was the special Russia one. If you can't trust the UK government here, than they could easily fabricate or claim they have hard proof of Russian involvement. You also have to ask yourself what hard proof of Russian involvement would even look like, and how realistic it would be to find such a thing even if Russia were to be involved.