That movie is good. It had solid characters and a plausible plot. It's a must have in any WWII movie collection.
Thinking about this more. . . since this involves what-ifs.. . what if Germany had coupled a solid special operation on selected British air strips or radar sites during the Battle of Britain? Small groups of trained German commandos with specific missions could be inserted into remote areas of England via subs. I concede that it would be difficult for these commandos to make their way to their targets but it is not impossible. As for getting out, they make their way back to a sub pick up point in another remote location.

Another thing, this spec-op is compartmentalized from the inept German intel gathering op to increase the commando team's chances.