There is really more than one question here: one is whether identifying a chemical requires the ability to make it; the other is whether the UK can reliably identify Russia as the manufacturer of the nerve agent.
The first question has a simple answer: there are plenty of known chemical structures that chemists don't know how to make, yet. This is common in natural products where chemicals are extracted from animals or plants and fully structurally characterised long before good mechanisms are found to synthesise them in a laboratory. So you don't need to be able to make something to know what it is.
But this is mostly irrelevant to the question of whether only the Russians can make Novichock agents. Not least because we know they have been made by others. But also because nerve agents are relatively simple chemicals and are easy to make (though not easy to make without killing everyone in the lab as a byproduct of the synthesis).
Chemical identification of a Novichock agent cannot possibly tell us where it was made: the chemistry can't tell us where the ingredients came from. The UK's portion down scientists have stuck to this line in statements.
This has led some to conclude that the evidence doesn't point to Russia. As does the fact that the alleged Russian work on the agents was never confirmed independently (other than the fact that others have confirmed that the recipes leaked by defectors actually work). But the allegation that this is some sort of false-flag conspiracy to damage Russia's image makes no sense. Firstly the development of nerve agents would be top secret and no government would blab about it in public. So we wouldn't expect the Russians to admit they had ever made or stockpiled the agents. Moreover, we would expect the western agencies to follow up the intelligence they had by both synthesising, testing and developing better ways to combat them. They wouldn't be doing their job if they hadn't. And we wouldn't be safe from hostile foreign powers.
So we should discount media simplifications that claim only the Russians could make them. But that isn't the point. We have good intelligence that they did research them and probably made stockpiles of them during the Cold War. So they have the expertise to handle them and use them. Some of the conspiracy theories allege that any competent chemist could have made them so why blame the Russians? But this ignores the issue of skill. Making a nerve agent isn't hard: not killing yourself in the process is very hard, deploying the agent is probably even harder. And it was the Russians who developed the agents and worked out how to deploy them. This matters, but conspiracy theorists have focussed on the ease of synthesis and ignored the extra expertise required to avoid death for all involved. They don't seem to understand chemistry: It takes large amounts of government investment to get that expertise.
So chemistry doesn't provide proof. But the chemical expertise to make and use the agents is held only by a very small number of players and the only one with a motive is Russia.
In short the key issue isn't chemistry but the expertise with particular chemicals. Plus the motivation to commit the act. The media should not claim that chemistry alone is definitive in fingering Russia, but the known-expertise plus history plus motivation makes a strong case.
Simple answers dealing with the chemistry out of context have already been widely appropriated to promulgate conspiracy theories about the Salisbury attack. Here is some of the context. Please ignore if you just want to stick with chemistry.
The real evidence that the Russians are the most likely suspect is a combination of the fact that they developed the agents (and therefore have the expertise required to work with them) plus the clear motivation they have to use them: it sends a strong signal that discourages defectors and opponents of the Russian government. They don't want to admit they did it but they want other defectors to know they did it. And they have form. A decade ago they used polonium 210 to kill another defector in London (unlike simple chemicals, polonium leaves a trace of radioactive debris that pointed very clearly to who used it as did the fact that only a very small number of countries can make it in the first place).