Interesting how Russia is responding to all of this. Any country not only accused of such a chemical attack, but having those accusations plastered on headlines throughout the world, would be pretty furious (and rightly so) if they were false.
Furor or anger aren't signs of strength in dimplmacy, quite the contrary. They indicate normally that nothing you can do. And Russia - can. Much of response or actions against British interests are non-public in such cases.
I would fully expect the British Ambassador to Russia to be called-in and given a carpeting, and if the accusations were not withdrawn, for members of the British diplomatic service to be expelled.
Diplomats have been expelled already. Russia expelled more British diplomats than British did - to eqialize the counts. And - it's easy to kick diplomats and ambassadors around, but it's very hard to rebuild the relationship later. Eventually dust settles, scandals go away, but it will take months of negotiations just to increase diplomat headcounts, and years or even decades to re-establish diplomatic relations. In this Western hysteria, it's not seen as important. Russians tend to think years and decades ahead. Their Western partners emphasize on getting re-elected, and lately - just get by without getting impeached or maidan-ed out.
To prove their case, the Russians would have done their own investigations to find out who, and what country, were responsible, and for that country's ambassadors to be called-in for an explanation, or a dressing down. If some other country were responsible for this attack, the GRU would be working on this case full-time until the culprits were exposed. The accused agents did travel from Moscow, after all, and unless the GRU is completely incompetent, it wouldn't be difficult for them to find out exactly who they were and where they came from.
They are working on this case. Skripals are their citizens after all. It's just that they don't go accusing to the right and to the left. And it's too early for alternate results, it's not done in a day or two. Plus, British are refusing to cooperate or provide any info - hmm, why would they not, if they are so right? How about the following - "here is what we have when issuing their visa, keep us posted as you keep finding things!". But no - the answer is "nothing will be provided". Perhaps they have something to hide they do not want to be discovered? Normally such things are being thoroughly investigated and every detail checked by teams of investigators from each and every country involved. After all - ever wondered why do these things happen only in London? Former spies live all over - Europe, Canada, USA. But stuff a-la James Bond happens only in London. Russians are the best at waiting and knowing when to strike.
I would fully expect Vladimir Putin to be banging his shoe at the UN until Russia was cleared, after all, his country's reputation is being exposed for what it is around the world. What do we have instead? Mild denials, "It wasn't us, must have been someone else", without offering any sort of evidence whatsoever. "The dog ate my homework" sort of defence. The British police have gone over the evidence with a fine-tooth comb, and come to conclusions which nobody is denying. The only argument on the Russian side seems to be "prove it", without offering one shred of evidence of who they think might be responsible.
Pretty damning in itself.
Yes, a lot can be done. But - as I mentioned, once things are done, it's not quick and easy to undo them. They can expel diplomats, enter trade war, exchange some tough talks even - sure. One thing that British seem to have forgot about - is that Vladimir Putin is one of 2 men on this planet capable of wiping that island, with its Theresa May, Skripals and all their problems from Earth's surface in 15 minutes. Even his predecessor Khruschev told them bluntly right in their Parliament about 50 years ago that they aren't unsinkable. Do you need to get to such extremes? Putin thinks globally, and does not want Russia to have reputation of someone who bangs shoes. Let Britain and America gain that reputation, and he'll ripe benefits of it. That's what he is after.