Perhaps the British spy voluteered to be poisoned... that would be an unbelievably stupid career choice. More likely he's expendable in the view of the authorities like a vagabond on the streets but from a public relations perspective valuably connected to Russia.

If the UK/US is behind this (which is a near certainty based on their behavior), it is useful for a media scandal that Skripal lingers on for awhile before dying so the "story" may be prolonged and fanned, and so there remains the option down the line of blowing up the scandal by finally having Skripal die of the poison or even having the Russians come and finish the job.

But anyways the "story" did its job, and now Theresa May and the authorities will try to wiggle out of admitting they deliberately falsified the event for political purposes (which sounds better than wiggling out of admitting to an outright lie and hoax which the Salisbury incident really is).

Remember the "highly likely" "indications." This language allows them to backtrack without totally losing face. Only their professionalism and competence is called in question, not their patriotism, so it's ok. They're not elected, they don't get paid to do their jobs. They're elected because they manipulated the voters.