It's obv
It's as obvious that we aren't directly (or indirectly) threatening nuclear action
If Russia placed the same ultimatum on the UK, there'd be plenty yelling about the irresponsibility of a nuclear power doing so. One could only make such a statement if they never put themselves in the shoes of the other side, as an exercise in understanding. Imagine this for a moment - that Russia might know they didn't do it, and they receive this ridiculous ultimatum. They ask for samples of the evidence, and are told they cannot have them. How should they feel about the UK then? "Oh those adorable, harmless Brits with their honest, always noble never-scheming MI6 and Foreign Office"?
as it is that the Kremlin was behind a Nerve agent attack on citizens in our country.
No, actually it very much isn't. It's just one of several possibilities at this stage. It could be true, or it could not be. And neither you nor I know it. It's called belief. And beliefs affect religious and non-religious alike.
Russia has been the belligerent country surreptitiously throwing its weight around at every opportunity
That's your perception. I don't doubt that you believe you are being honest about it. But I have to wonder what you know about topics like Victoria Nuland and State Department and CIA involvement in the Ukraine coup, and CIA authorization and support for civil war against the Donbass people instead of patience and talks with them to formulate a Ukraine acceptable to both sides (pro-EU/pro-Russia), and the disregard for decades of upsetting the ballistic missile balance (you can nuke me/I can nuke you) with anti-missile shields popping up all over the place close to Russia, the US withdrawal from the Anti-Ballistic Missile Treaty in 2002, the broken promise of the West (to Gorbachev) that NATO would not grow eastwards, UK/US support for ISIS and others in Syria, and a whole host of other issues which will concern Russia very much. Sanctions are typically a prelude to war or total submission, and Russia is unlikely to submit.
Declarations by ex-CIA directors admitting that the West also interferes in elections and arranges coups and the like, but it's different because "we are the good guys", are unlikely to reassure Russia, who are therefore the "bad guys".
and then denying any involvement when the truth is patently obvious.
How often is it (obvious) in this world full of lies? Do you know that the, or one of the, inventors of Novichok - Mirzayanov, has been living in the US since the 1990's? Do you believe that the UK government isn't lying about not having any biological weapons, when they lie so often (as do all governments) and that's something
really high up the list that they
would lie about? No, Russia aren't the only ones with the nerve agent.
It's about time it received some tough talk itself, backed up with reasonable action consisting of whatever the circumstances demand.
Well if that is what would impress you, okay.
What would impress me is the prosecution allowing the defence to see the evidence with which the defendant is being accused, which would be rather more British (as in what once made Britain truly great, not the Perfidious Albion variety) than "haha you can't see it, guilty as accused and that's it." Imagine if you were tried like that just for shoplifting. You'd declare the fix was in. And this is far more serious (for the entire world not just the UK and Russia) than accusations of shoplifting.