The Germans never got a commando unit into Britain for an attack. “The Eagle has landed” was complete fantasy as is “Churchill, the Hollywood years”. In fact they had trouble just getting covert spys in to Britain, let alone commando units.
The reverse however was quite different.
The big difference being the Germans were not liked by the vast majority of people in the countries they occupied. Thus the British received massive amounts of help from those occupied countries who would house, feed and hide incoming commandos and agents. In fact many of these “invaders” were from the nation they were infiltrating further frustrating the Abwehr, etc. because they could blend in alot more effectively than a German in UK.
On the flip side, the Brits weren’t to keen on the Nazi’s in general so if a stranger arrived in UK they were most likely to get arrested in short order. The British, even in large cities, were very familiar with each other so a new person, with or with out strange accent stood out like a sore thumb. Even Poles fighting in the RAF were occaisionally investigated.
This is also proven by the sad lacking of successfull German escapes from POW camps, they were nearly always rounded up by the second night.
One of the measures conducted by the Brits to slow down invasion was to remove signposts and place names. Thus anybody asking where they were or a way to a place automatically were under suspicion, many locals would not give this information out. I’m pretty sure that it was proven that practically every agent they dropped into Britain was captured with days and in most cases turned and used to send back false info back to the Germans. Some even gave themselves in!!!
See http://en.wikipedia.org/wiki/Twenty_Committee
Many of the agents who reached British shores turned themselves in to the authorities. Still others were apprehended when they made elementary mistakes during their operations.
So basically the Germans never managed it. A point to note, is that although the Germans were good at the old blitzkreig they were babes in arms to the British when it came to Int gathering/counter intel and general deception operations. Not only did we have their entire spy ring in our hand, we (with the poles) had cracked Enigma early on and of course Op Quicksilver and the Phantom Signal Regiment meant they didn’t know if we were coming or they were going!!!
One film that is true (or based on truth) was the story of the Brits using a man who had died of pneumonia (in an autopsy the symptoms of pneumonia are simialr to those of drowning) to be a “special” courier. He was launched off the back of a submarine and was handcuffed to a briefcase in which invasion plans were placed. This convinced the Germans that Greece was to be the next invasion point.
The film was called “the man who never was” and depicted the true story of Op Mincemeat (I think that is the real Op name as well).