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Obviously, whomever killed Litvinenko wanted others to know what they did, even if they didn't necessarily want them to know how they did it. Personally, I think the sophistication of the method suggests that it wasn't sponsored by the Russian government. Their economic ties to the west are too large and tenuous to risk being blackballed by using a method so beyond the pale. It seems more likely that they would have simply commissioned a more traditional hit--just as effective and about 99% as good of a message, but with a significantly lower potential for backlash.
One of the assumptions that this story is based on is that Large is an expert of polonium biophysiology. Since he says he's not, I would take a good bit of his conjecture with a grain of salt. I trust him when he says that this polonium was produced in a reactor, I do not trust him when he says that it would have to have been inhaled. The former is his area of specialty, the latter is not. A polonium organometalic compound could might be just as easy to transport, but infinitely easier to deliver.