There will, I suspect, be a lot of the usual goalpost-moving as this story develops. At some point maybe you'll be lucky enough to see the evidence, but then you would say that it was fabricated or the test was done badly, so you'd demand to do the tests yourself. Then you would reach the same conclusions, and decide that the sample must be faulty. But no, that too is genuine, so then it becomes clearly a false-flag operation...
Obviously we have only our government's word on all of this, and I am not for a second advocating blind faith in what they come up with. But there is a difference between healthy scepticism and rampant conspiracy-driven paranoia.
Whom do you trust more? That's what it comes down to in the end. In a dispute between, on the one hand, the British government (and then all the allies of the same who have come out in support of their position), and on the other the Russian government, whom is more worth taking seriously?