The UK will, of course, apologise if Russia provides compelling evidence that it was not responsible!
With folks like this who rely upon their ability dishonestly to deny their actions, you just have to invert everything. Assume guilt rather than innocence and put the burden on them.
We assume it is your troops in Ukraine unless you can show us they belong to someone else.
We assume you interfered in the US election unless you demonstrate you did not.
We assume it was your nerve agent unless you demonstrate that it was not.
Reputation counts.
Smirking, as Putin did when asked about the poisoning, is not the action of an innocent actor.
And he's not invulnerable.
His world cup may disintegrate before his eyes. He may find credit is harder to get in London.