On the question of how Mosley and the BUF would have behaved in the event of a German invasion, see
Illusions of Grandeur: Mosley, Fascism, and British Society, 1931-81 by David Stephen Lewis:
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Of course the assumption that Hitler would
want Mosley to rule the UK in the event of a successful invasion is questionable. As John Lukacs notes in
The Hitler of History:
"All over Europe (for example, in Holland, Denmark, France, Romania, and in a few remarkable instances even in Austria), local National Socialist leaders were abashed when they found that Hitler did not support them and paid them hardly any interest at all. He preferred to work with the established pro-German governments of such provinces and states. The most telling example of this occurred in Romania in January 1941. There the National Socialist and populist Iron Guard (whose anti-Semitic ideology and practices were perhaps the most fanatic and radical in all Europe) got into conflict with the nationalist and military government of General Antonescu, whom Hitler respected and liked. When in January 1941 fighting broke out between the Antonescu and Iron Guard forces, the Germans unequivocally supported the former at the expense of the latter, on occasion with German armor and tanks.
"Of course he had his reasons. While the war lasted, he needed order in the countries that were his allies or satellites--a kind of stability that must not be endangered by revolutionary experiments, and that assured undisrupted deliveries of necessary material supplies to the Reich. Thus he put up for a long time with allied chiefs of state—a Petain, an Antonescu, Regent Horthy of Hungary, King Boris of Bulgaria—some of whom he knew were not wholly loyal or unconditional adherents of a National Socialist Germany. Still, it is significant that he did not offer the slightest promise or give the slightest indication to the effect that sooner or later, perhaps after the war, his foreign National Socialist followers would get their rewards.* [FN] He would, of course, recognize and support some of them in 1944, when his former satellites or junior partners deserted him; but that was no longer important."
https://books.google.com/books?id=oRwJs6qCMvIC&pg=PA162