What if instead of full naval invasion, The Germans decide to deploy Commandos under cover of air raids to sabotage the British war effort at the home front? How much damage could such a move wreak before they were caught?
A lot of nuns are going to be shot. I tried to find a suitable Dad's Army clip, but couldn't find one.
But my serious answer is organisations like the Home Guard were protecting the sort of targets the commandos would try to attack so they probably don't do much material damage. However, they might damage morale, depending on how successful they were, but if they were mainly failures British morale would probably be higher. There might be some lost production due to stricter security measures and probably more than the material damage caused by the raids.
It also depends upon the number of men dropped on each raid. E.g. if they land enough overcome the guards on a railway bridge. But that probably means landing companies or even battalions of men in concentrated groups instead of scattering smaller groups over a wide area.
But AFAIK the Police and MI5 were very effective at capturing the spies and saboteurs that the Germans did send IOTL and many of them were turned into double agents.