It has often been noted that the twenty or so spies (of various nationalities) sent over to England and Scotland in 1940-1941 - some by boat, some by parachute - were very poorly qualified, prepared and equipped. All were collared within a day or two, generally as soon as they made contact with the population.
In the atmosphere of general suspicion at the time, with everybody on the lookout for spies and fifth columnists, it could hardly have been otherwise. The spies did not know that road signs had been removed or painted over and some immediately drew attention to themselves by asking people where they were - in an obviously foreign accent. Others made elementary mistakes like cycling on the wrong side of the road, or asking for a drink in a pub outside licensing hours, as noted above. Almost all were carrying articles on their persons that could easily be identified as German. Only a couple had been provided with addresses of people who might be able to help them - and those people were already well known to MI5.
It would seem that all this was in fact due to deliberate sabotage of the espionage effort against Britain by a group of antinazi officers in the Abwehr. The case is convincingly made in a recent book by Monika Siedentopf: Unternehmen Seelöwe - Widerstand im deutschen Geheimdienst. Perhaps it will be translated into English.
The central figure was (naval) captain Herbert Wichmann, who was in charge of the Abwehrstelle Hamburg, which was responsible for collecting intelligence about Britain. With his co-conspirators, he carefully selected agents whose chances of success were minimal to nonexistent from the beginning, in view of their intelligence, character traits, motivation, language skills and knowledge of Britain. These agents were then "trained" by instructors who (while not themselves part of the conspiracy) were similarly unqualified for the job.
For example, the Dungeness/Lydd spies, with some others, received a few lectures on the British army (to enable them to identify units &c.) from an interpreter, a certain Werner Unversagt, who spoke good English but had never visited the country and had never had any interest in the British army until then. Nor did he have any other military experience, or experience with intelligence work, so it is hard to believe the spies could have benefited much from this instruction.