I have not yet watched SS-GB and I don’t think it’s quite my sort of thing, but raises an interesting question or two. I have heard of ‘counterfactual history’. That is writing about things that might have happened if things had turned out differently. Say, if Harold had won at Hastings or if Labour had won the 2010 General Election.
To some historians, this is totally pointless. We study what did happen. There’s enough to get stuck into, without wasting time on what did not happen. The big argument in favour of doing some counterfactual history, (or speculation, if you like) is that it helps us see the importance of what did happen. For instance, if the South had won the American Civil War, would slavery have lasted until now? Would other states have left the Union whenever the Federal Government did something they did not like? If the British had not ruled India, how long would the rajahs have resisted modernisation? Would Gandhi have been a rebel against India?
Counterfactual history can also help us see how overblown something had been. If things would have turned out pretty much the same in the long run. If Scott had reached the South Pole before Amundsen, would the World be any different now?
As a writer of historical fiction, I am between the two approaches. I frame my stories in real history. Or try to. But I fill in the unknown parts with things I have made up. Things that I think could have happened. I tweak the facts a little at times to make the story flow better, but I try to be faithful to the actual events as far as possible. I include historical notes in my books to help the reader sort the facts from the fiction.
Sometimes, a look at counterfactual history helps me get a better perspective. Think what a Jacobite victory would have led to.