I know why Russia didn't respond to UK's questions:
Russia could easily have sent over their ambassador:
"That nerve-gas is not from Russia and Russia has no idea where this nerve-gas came from. Maybe it was some rogue ex-soviet scientist or somebody else, but we can't help you. Sorry."
How hard is that?
Instead, Russia is attacking the messenger and demands to get a sample of the nerve-gas for analysis.
Why?
Because the russian government doesn't know what to say, what to admit and what to deny.
What if the russian ambassador or Putin explicitly deny involvement and then later there's evidence implicating Russia?
What if Russia doesn't know whether russian assassins tried to kill Skripal with Novichok because those assassins haven't reported back yet?
Why does Russia want a sample?
Because there's several kinds of Novichok and if Russia knows which one was used, they can look up whether russian assassins are using that version of Novichok or not. And then they can tailor their response.