Mosley stated in his memoirs that if offered a role in a Nazi government he wouldn't accept. Take that for what it's worth.
I think the Germans would have a hard time finding willing collaborators. And collaborators having a hard time staying alive, I see lots of assassinations as warnings about making a poor choice.
I think someone would have to be brought in...
While I do agree the nazis would have a harder time finding collaborationists in the UK, I don't think it is that hard. The nazis had a lot of silent supporters in the british upper class. Quite a few members of the house of lords were under severe suspicion during the war, and only avoided arrest due to their noble standing. Important men such as the duke of Westminster, maybe the richest man in england back then, were radically anti-semitic and defended friendly relations with Germany.
Also, when talking about Mosley, Lloyd George or any other major politician we suspect could cooperate, we should consider that he never really had to face that choice. For instance, Pierre Laval never gave a solid hint he would be a collaborator before the fall of France. Sure, he was against the war, but he declared during the "Phoney" period that now that conflict was upon France, they should fight it with all resolve to the bitter end (something these british politicians never did). Six months later he is the PM of a collaborator government.