The way I look at it (my opinion) three things are involved:
1. The UK government collaborating the USA government, in a precedented effort, to frame the Russian government into more sanctions and economic/political marginalisation. Or,
2. The Russian government deliberately poisoning Sergei Skripal for fear of military intelligence sabotage.
3. The British government deliberately poisoning Sergei and his daughter(...) for fear of British intelligence sabotage.
All points are in order of preference.
4. The British secret services do this in collusion with Russian secret services to pursue their obscure goals.
Like: give US pretext for sanctions which will (according to Bloomberg) cause flight of Russian-owned capital from US
with some dollars maybe sticking to City boys' hands.
Or discredit Theresa May and push harder for Brexit. Or whatever.
There could be a lot of intermediary moves between the poisoning and the desired effect.
A substance that causes heart attack and is not detectable in the body after a few hours. And which doesn't belong to military poisons class.
Don't you see chemical weapons - White Helmets fakes in Syria and missile attacks - alleged WMD in Iraq connection?
By all this talk of chemical weapons, US try to hide their bacteriological military bases in Georgia (ex-USSR) and probably other places.
Apart from that, whenever ISIS forces in Syria were under most threat, the Skripal story was fanned into fresh fire.
UK+US+EU invested a lot into ISIS, so losing this asset was a pain.