There is so much C20th literature and pulp fiction around the hypothetical of 'what if the Nazis had won?'
The Man In the Castle by Philip K. Dick, Peace In Our Time by Noel Coward, and SS-GB by Len Deighton, and - more recently - Weaver by Stephen Baxter. The more interesting question, for me, though, is 'would we resist?' I'd like to think so, on a personal and societal level!
Here, though, it seems that history rests on a different turning point; not Operation Sea Lion - the planned Nazi Invasion of Britain scheduled for 1940 before Hitler turned his attention to the Soviet Union and Operation Barbarossa - but rather whether the British stood and fought under Churchill or, and this was a serious idea at the time amongst some, whether the British collaborated with the Nazis in order that war against a then-superior military power may be averted, whether in the name of peace (in our time) or the Nazi-sympathetic, far-right politics of Oswald Moseley and the British Fascists.
And it is that latter turning point that seems to be the cause for the divergence between the two worlds in Inferno, with the novelisation by Terrance Dicks explicitly referencing that moment.