Thank you Martin for considering my idea. I do not present it as truth, but merely as what I consider to be a plausible idea for others to consider.
I do not suggest that the security services knew that Yulia planned a suicide.
Any sort of attempt at murder by poison is not guaranteed to work. Yulia failed because she was not a professional assassin, but even professional assassins fail.
Fentanyl is not a non-lethal poison. It is an extremely dangerous and very toxic drug. But it does not act in the way that a nerve agent would.
John Helmer, who is based in Moscow, has highlighted on his blog the fact that the the British defence ministry are not able to provide a genuine chain of custody for the Skripals blood samples from the hospital to the Porton Down lab and that therefore their assertions regarding the agent that poisoned the Skripals are entirely worthless.