>As far as I remember, it was still possible (in terms of banking systems/santions) to pay the gas in Euros as usual; Russia decided, unilaterally, that they would require the payment to be in Rubles (to try and limit its plummeting value)...
That's a bit disingenuous, ins't it?
Maybe you're right and EU/UK could still have paid in Euros but, with Russia being frozen out of the world's banking system, they [Russia] wouldn't have actually been able to access the money. So it pretty much amounts to the same thing. They'd be giving the gas away.
If you were working on a job for someone and suddenly had your bank accounts frozen, it wouldn't be unreasonable for you to tell the person employing you that, while you were happy to keep on working, since you couldn't access your bank account, you'd need to be paid in cash for the rest of your time.
And you'd rightly feel aggrieved if that person refused, but then put it about that you were an unreliable worker who'd walked off the job.