But we aren't trying to go to war here.
I am struggling to see what sort of ulterior motive the UK could have for apportioning blame without being as sure as is reasonable.
I suspect the "evidence" is to a large extent technical and chemical, as well as the very powerful circumstantial.
It isn't as if the general public would know chemical formulae for this particular class of agent and be likely to go "ah yes, of course!" and presenting whatever such evidence, apart from being likely to interfere with the analysis by independent scientists, would in any case be unlikely to satisfy anyone who wants to believe otherwise, because tossing conspiracy theories around is just too easy.