Apart from the perpetrator(s) and any collaborator(s), nobody is likely to know exactly whodunnit. In the UK it is possible (although I would not assume it) that investigators know more than is in the public domain, and (as already said) the source of the poison may lie outside Russia. I would not rule out British involvement in the deed although I would like to think it unlikely and, one way or other, Russia certainly seems likely to be behind this - for which they can really only blame themselves because of their acquired "form". Maybe the real truth will eventually come out, although doubt is likely to linger forever because nobody will admit to their part in this. Also, the truth is likely to be massaged for public consumption and advantage on both sides of the current stand-off.
The repetition of an untruth until it gets believed by the majority of those hearing it has been going on forever, not least within the UK.