What happens if the U.K. concludes Russia poisoned Sergei Skripal?

Skripal in Moscow District Court in 2006. He was convicted of providing information to MI6. Photo: TASS\TASS via Getty Images

Former Russian double agent Sergei Skripal and his daughter remain in critical condition after being poisoned with a nerve agent in the English town of Salisbury on Sunday, and suspicion has naturally fallen on Russia.

The bottom line: Senior members of the British government have said they aren't ready to "point fingers," but that there will be a robust response if a "state actor" (ahem, Russia) is determined to be the guilty party. The question is when such a determination will be made, and what that response would look like.