When considering the implications of a fascist Britain in HOI4, folks need to keep a couple of things in mind:
1) Flag changes for all countries, even the majors, means that puppeting powers gives an appropriate flag. This is a good thing.
2) In HOI3, the only country who could change flags was France and Spain, and even in those cases, it was because a whole new country was created out of thin air, not because the same country got a new flag. Now we can change flags whenever it would make sense. I shouldn't have to remind anyone how useful flag changes would be for Germany at the end of the war, Korea, China at the end of the war, and Italy.
3) Modders have been given a huge blessing. I can only begin to see the modding possibilities of such a mechanic in an HOI game.
4) A lifting of the "faction leaders cannot be couped" mechanic would also be a good thing. While I will be the first to argue that the historical government of Britain wasn't about to become fascist (or even close to it), I'm also holding out hope (based on DDs) that someone can run the British government into a trainwreck that would allow other political possibilities. It would be nice that even faction leaders would have the threat of a coup hanging over their heads, making decisions about political party ORG and popularity more meaningful. "Prime Minister, we can't do X. It will give the BUF an opening to demand seats in the Cabinet! And from there, we will face the recipe for a coup, because Oswald Mosely wants to be Defense Minister!" Keep in mind that some players may want to run amok as an ahistorically fascist UK, but other players may want to avoid becoming facsist, and find themselves having to make some hard choices.
Hard choices = fun in a grand strategy game