As long as we're on the Soviets, I nominate Michael Emerson (Ben from Lost) as Beria.
I disagree over Churchill.
Albert Finney was the best screen Churchill I have ever seen in "The Gathering Storm". He got it spot on. Name is a link to it on Youtube if you doubt me.
hows about Paddy Mayne? Got to have him in it. And Major Howard too. Lord Alexander-possibly Stephen Fry.