Yep Id lean more towards the odd unit of volunteers rather than a whole legion. In the first place as has been said Hitler's aim is to keep Britain as an ally, but would rather it be a weak one at the moment, so its enough force to prop up the Fascist regieme- not enough for an outright win.
Secondly things are getting a little busy for Adolf elsewhere, having had his excersion to Austria, he may shortly need boots on the ground elsewhere, and he wants the German African possesions back- another potential cause of difficulty if he is over helpful to Mosley. The German Army isn't as big as we were giving it credit for, so there is a limit to commitments, particularly of specialist staff like Pilots and Tank Crews.
It might also be possible that some volunteers that turn up do so of their own accord, maybe even to get away from Adolf's regieme. We've set up the 'Duke of Brunswick' as one such character, German Aristocrat, with a following of Freikorps -ex servicemen who has fallen out of favour with the Reich and think its prudent to spend some time abroad. Given the involved blood lines of European Royalty it doesn't take much of a leap of faith to see why such a German might turn up at Edward's court and offer his services