No one. An ice pick, a bullet, a lethal dose of ricin, would all be Chekist-style. The Russian mob would probably uses something equally cheap, but a bit more gruesome.
The amount of polonium, Litvenenko's Chechen connections and conversion to Islam, the possible use for polonium in neutron sources needed in the triggers of 'suit case' nukes, or even--given its toxicity--its utility as a component in a 'dirty bomb', all point away from the assassination theory, and toward a smuggling operation in support of Islamist terrorists operating against the West.
But it's the UK, and handguns are illegal. Hence, the use of an alternative.