The fact the the fancy poison to kill Mr. Litvinenko was worth $4.5m makes me wonder that it was actually an assasination.
More likely an accident in a smuggling attempt.
The reason that the KGB used polonium-210 is that the British would be fooled into thinking that it was thalium. The British were giving Litvinenko the antidote for two weeks and it wsn't working. He should have died within two weeks because the dose was huge but he survived an extra week due to his extensive exercise regimenHandling it does not effect a human adversely but drinking it means that the person will die and their sweat will leave markers wherever they go.
Litvinenko was murdered because he named the KGB as the sponsors of al-Qaeda and all the terrorism in the world from The Jackal to al-Zawahiri and because he went on Russian TV and accused Putin of crimes. He was working with Berezovky and others who owned TV stations that weren't government controled to maintain the little bit of democracy that existed under Yeltsin. Sadly they failed.
If it was a smuggling operation why would Litvinenko drink it? Handling it doesn't kill you!