"You don't earn a high level assassination attempt like this just because you were a spy a couple of decades ago, particularly if you have already done time in Russia for it and were voluntarily exchanged."
I think I read in the telegraph or the FT that he had been giving lectures in general Russian modus operandi, so still assisting British intelligence. Not about current activities, presumably, as he was out of those circles....presumably.
But you have to wonder about the comings and goings of Yulia. That she thought she was safe to go back and live in Russia after her father was swapped, to come and go without surveillance.
Disclaimer: speculation and conspiracy theories:
What if Skripal wasn't actually the victim but the perpetrator? What if he had been swapped as a double deal - to spy here? Coming to live so close to Porton Down...his Dd bringing ingredients and instructions?
The only way I think the Americans would be implicated would be if again, whist he was in prison he had been re-recruited to Russia, sent here to wreak havoc against the U.S elections and now the Americans have attempted an assassination that looks like the Russians. Orchestrated from within Porton Down between the CIA and MI6.