Some further thoughts on this. Up to now in my theory I have tried to avoid implicating the two Russian men Alexander Petrov and Ruslan Bashirov.
However, and while I still do not think they were the poisoners, it occurs to me that they could have been the people who supplied Yulia with the drug (Fentanyl) and that they could have left it at a designated spot on 4th March 2018 for Yulia to pick up. (No questions asked). She then put it in a perfume bottle she had in her handbag.
They told RT about their occupation in their famous interview with Margarita Simonyan in September 2018:
“In short, it’s the fitness industry, it is about sports nutrition, vitamins, proteins, microelements,” the men claimed. The pair added they would rather not elaborate on the details for one reason – to protect their clients from unwanted attention.”
Their description does suggest they would have had detailed knowledge of drugs and where to obtain them. Their coyness might be an indication that they are a bit dodgy and prepared to supply illegal drugs to customers if the price is right. They could have supplied the Fentanyl to Yulia without having any idea what the drug was going to be used for and may still have been very shocked when they found out about the poisoning.