Richard at the Flicks makes a number of interesting points. However I would like to comment on two of them.
The harbour scenes were shot in Charlestown in Cornwall just a few miles from where the two dentists actually sailed. While making the film the elders of the village pointed out that although harbour lights should not be shone at night, their village kept them on during most of the war. There reasoning was that no one had ever bombed them and nobody would. That part of Cornwall was not bombed and was out of way of most German flight paths.
Also from a purely technical point of view if there had been no lighting in that scene the audience would not have been able to see anything.
When King & Cuthbertson actually landed in France there was no enemy along that particular stretch of the coast. This was well documented in newspapers at the time. For the Germans to patrol every bit of Northern France would have taken hundred's of thousands of men, men who could not be spared. The crossing at Cherbourg was long and few at the time thought this would be an area that soldiers would land. (On their return to their boat they did encounter a German officer, as can be seen on the deleted scenes on the DVD).
Also it was 1942. Up to that point the war had not been going well for the British and an invasion of France at that time was not considered feasible both by the Germans and by the British. The Germans were convinced that it was only a matter of time before Britain fell. The British they thought were no threat to the mighty German Army.