No - judging by what I've seen, it couldn't have solely been a UK-Russian deal. My understanding is that the US certainly played a part in it. At minimum, the FBI cooperated with us, which they could've only done, as a domestic-based organisation, if ordered to by a higher US authority ....
I think this report confirms it:
http://www.bbc.co.uk/news/uk-43315636
The UK will certainly take some sort of stance against Russia, once official confirmation of a Russian origin for the nerve agent is released. We're told it was a 'rare' agent ... meaning it may well be possible to track its exact origin, maybe even the very laboratory which produced it. What official response we come up with may not be much, in international terms, but it will be something.
Perhaps we'd do what Trump has attempted regarding N Korea, and call upon other powers to ally with us in a common cause ... maybe yet more sanctions ? Or, maybe we'll order Russian diplomats out of our territory. I don't know.
Anyway, our media tells us that UK-Russian relations are now in 'a deep freeze'. In fact, the BBC aired, just minutes ago, a commentary-programme suggesting that Russia was heading for diplomatic isolation from the West as a whole.
I think that Russia really did mean, primarily, to send an 'oppose Putin, ever, and see what it brings you' message to other Russians. I think it was simply that. However .. Russian arrogance is such that foreign lives mean very little to them. Therefore, they've no human constraints upon the method of execution they use. Besides ... the greater the loss of life, the greater the media coverage will be of it. Russia, in sending its message, may rely upon that extent of coverage, and so will work to maximise it.