The Russians can determine where it came from. They kept meticulous records on every batch they made, and where it was stored. Many batches were stored in old Soviet Bloc countries.
Each batch differs slightly, and has a different signature.
This is all open source info on the web...
Did Britain provide Russia a sample, as requested? And, as dictated by the Chemical Weapons Treaty?
Not that easily.
https://www.google.ru/amp/s/zona.media/news/2018/03/30/reprink/amp/
You can read this article in Russian using Google translator.
It explains agent proliferation.
Many entities could do it.