I must say I kind of agree with Throbert McGee but I don't know enough about chemistry or medicine to argue the case. I think that logic still points at Russians as the main suspects.
I suppose the suspects in order would be:

Russian spies
Some criminal corporation / interest that Litvinenko upset
British spies
the most bizarre suicide ever....

Not that I know a lot about Litvinenko but it's hard to imagine a good reason why the UK would like to kill him? After all, he was on "their" side...

Depending on who actually did it, it's either a huge cock-up or rather successful... making it a very good warning for those who actually know the truth about what happened. What a horrible way to die! Nobody is going to upset that group/organisation again if they can help it!