Mosley would have absolutely been the first point of call for the Nazis, since Hitler knew of him by name and greatly admired him.... and, you know, was at least a guest of honour at the mans wedding. Oh, and directly funded the BUF. Oh of course Mosley bemoaned the fact that any dared suspect that he'd be a collaborator. But if German troops never set foot on Belgian soil I'm sure people would be utterly scandalised at the slanderous slander that a great socialist like Henri de Mann or a bold charismatic patriot like Léon Degrelle would collaborate. And heaven forbid that anyone suggest that Charles Murras or Petain, the Lion of Verdun, would collaborate with the Germans - they're true Frenchmen who hated Germans!
As to the belief some people have that the Germans are just so innately sensible that they'd want a more establishment figure, remember this is the same German government that arbitrarily named Quisling Prime Minister of Norway, right in the middle of their negotiations with the real government of Norway for surrender. Mosley is popular enough with the top Nazis to be propped up without question or thought for the consequences.
So yes, the Nazis would absolutely offer Mosley power, and likely do so via the restoration of King Edward for legitimacy - and I don't really doubt that he'd accept. After all, he's not being appointed by Germany, you silly communist terrorists, he's been appointed as per tradition by the King! He'd be helping things from getting worse of course, after all, he was the great genius who was always ready to save the nation - he'd make sure the Germans were nice to us, with his power and intellect and brilliance.
That people think otherwise is just proof of the creeping cult of Mosley apologetics disturbing influence.
And while yes, it might make sense on some abstract theoretical level that fascists be part of a resistance to German occupation because of patriotic sentiment or what not, this was pretty damn rare in Europe and most fascists who went into the various resistance movements sort of stop being real fascists at that point, and I don't see what makes British Fascists so different.