It happened Here (1965)
Chosen by Darron Bowley who had this to say: 'This film postulates the 'what if Germany won' scenario by examining it through the eyes of one woman. Filmed in an almost documentary style it show a bleak vision of England under Nazi rule. It's notable for the sequence where the woman is talking to a bunch of English fascists. Apparently, this was ad-libbed and they are expressing their own opinions in their own words! Insightful in that the film suggests that fascism can breed anywhere given the right circumstances, a poignant point given the current rise in extreme politics/religion around the world. Perhaps a better title would have been 'least we forget...'.'
This doesn't quite hold together strongly on it's throughline but is none the less a compelling 'what if'. Not exactly a 'mockumentary' though filmed in strong raw docu style, with clear influences from Soviet and German propaganda movies, it posits a world in which Germany successfully invaded Britain and holds a power in the simple images of their soldiers marching in London. An incongruous and uncomfortable image backed with neat, small details that make it believable (the acting is fairly terrible for the most part, but that adds to it's documentary charm) and easy to see how the majority of people would go along with it, not out of cowardice but through simply wanting to keep in motion, to not stop their lives.
It has a similar problem to the Robert Harris book and adaptation Fatherland in that it's not exactly a surprise to an audience that the Nazis are killing undesirable persons but this works better by having English nurses carry out the deed and the way they justify it to themselves is horrifying.