Since the topic came up in the discussion of my Sealion Vignette, let's add a more serious version of the subject here.
Imagine a timeline in which Sealion is attempted and the invasion defeated. What happens next ? In particular, what is the impact on popular culture. The way that events are portrayed in popular culture depends not just on the events themselves, but also on the mythology that builds up around those events. An example of this is the way that the Little Ships of Dunkirk are portrayed. The ships did play an important role, but that role is sometimes exaggerated and the impression is given that the ships were mostly manned by their owners when most of the ships had navy crews.
The first step in the development of the mythology of wartime events is propaganda. While there are a number of ways that the events of Operation Sealion could develop, the way they can be spun in British propaganda films is a bit more predictable. One approach was to show how "ordinary" people were helping meet the challenges of helping the war effort - see
The Bells Go Down for an example based upon volunteers in the Auxiliary Fire Service. Another approach is to ridicule the enemy, and the Nazi master race mythology provides a rich target for humour - see Will Hay's
The Goose Steps Out.
This is where the Home Guard enters the picture. Obviously most of the actual fighting is going to be carried out by regular forces, but the Home Guard is going to be involved as well. They are probably going to be the first to see the Germans landing and the first to engage them in order to slow their advance until the army gets there. There's going to be stories of bravery and stupidity (sometimes at the same time). There's going to be drama, and comedy, and tragedy - in short, everything you need for a good propaganda film.
After the war, the myth building will continue. People want to tell their children about the conflict that's just ended. Operation Sealion is going to be a big part of the post-war British Psyche - as big as Dunkirk and the Battle of Britain were in OTL - and the Home Guard will continue to be the centre of the myth. Movies will be made building upon the stories and earlier propaganda. The portrayal of the Germans will probably get more nuanced with time - you'll see the Honourable Heer Officer or the Scared Conscript. At some point Hollywood will want to make a version of the story, and how they fit an American protagonist into the plot will horrify or amuse the British.
Eventually, after enough time has passed, there will be parodies of the Home Guard's role in defending the country. Probably not before the Eighties though. The reaction to
Blackadder Goes Forth indicates that maybe even the Nineties might be considered Too Soon by some people.