My understanding was that it was not so much that just the orcs were Nazis, as that Sauron was the embodiment of the Germany of British black propaganda and Tolkien's own Ludditist fantasies. Orcs existed well before the events of LoTR doing Orcish things, they just happened to be useful tools of Sauron.I thought the orcs were Nazis?
The book isn't a particularly good metaphor for anything in real life because while the Hobbits are indeed the embodiment of the simple English country folk, as The Shadow points out, there is no real representation of the British Empire, an entity rivalled for pure evil only by the likes of the pre-Russified Soviet state before Stalin cemented power. Hobbits aren't deliberately starving their colonial subjects to death and using the war against Sauron as an excuse.
'The Hobbit' is a better book than any of the trilogy.