If you make a feelgood film which is complete candyfloss, where everybody is good and everything is beautiful and hunky dory, it won't appeal to the audience.
The reason is that a military defeat of Britain will bring about the disintegration of the British Empire. This would not be of any benefit to Germany.
The flag that was the symbol of slavery on the high seas for a long time was not the Confederate battle flag, it was sadly the Stars and Stripes.

