To believe the Russian spies (we didn't do it, somebody in the UK is killing Russians, they are liars!) requires disbelieving the British spies (the Russians did it, they're liars!).
So it has the property of being believe the British government narrative or you're agreeing with the Russian government, which feels a lot like guilt by similarity of argument.
The default position for those who believe in the rule of law is to not believe the British narrative, because they are the accusers and the burden of proof is theirs and they have publicly provided only circumstantial evidence.
It's quite a clever trap that many educated people seem to have left unchallenged.
No it doesn't. It is quite possible that they both lie.
The British narrative is the Russians did it.
The Russian narrative is they had no part in it.
If you disbelieve the Russian narrative, you believe they did have a part in it.
It's possible to not believe the British narrative and not care about the Russian narrative because you don't believe they have anything to answer for, but that is altogether different from disbelieving both.
For what it's worth I don't believe the British narrative on this because of it is inconsistent with previous versions of itself and the evidence I have personally seen is circumstantial.
There is no need to take a side. It is possible to simultaneously distrust the British spies and the Russian spies.