Have a butchers at this:
wow, im surprised that they couldnt accomplish at least one mission of blowing something up in London…it seems pretty easy.
Given the quality of the spies Germany was sending, that’s hardly surprising - from memory one was caught after walking into a pub at about 10am and asking for a pint of cider or some such. In a heavy continental accent. Somewhere extremely rural right on the English Channel.
The German intelligence agencies weren’t really much good in the UK in WW2, while British counter-intelligence was superb.
We picked up almost all their agents soon after they landed and then turned them. They were given a simple choice - send what we tell them to properly, or hang. Most of them were sensible about this, but not all!
Not only that, but we turned every agent and carried on through the war making the Germans believe that they were still agents. Often feeding them true snippets just to keep them on the hook.
If only our present-day spooks were so good. 
Believe me, they are!
As previously pointed out by Stoat and pdf German attempts to infiltrate Britain was an utter disaster. I attended an interesting series of lectures, part of the fifty’th anaversary of the May(1941) Blitz on Liverpool one of which covered the counter intelligence war.
It was a popular myth that German agents either parachuted or were landed by U boat along the East and South coast, in reality Most agents landed in the Irish Republic and and travelled to mainland Britain via the Belfast or Dublin ferries to Liverpool with the support of hard line republicans (IRA). Once in Liverpool they would stay a B&B in the city centre run by an Irish woman who was a known IRA sympathiser before attempting to travel south by rail from Lime Street Station it was at this point that most of them were arrested.
In reality most of the German agents had been compromised as soon as or soon after their arrival in the Irish Republic. Elements within the Irish Govenment and also within the IRA realised that given Irelands strategic position Hitler would not honour Irish nutrality should Britain fall and that the Free State would become another occupied territory consequently intelligence on the arrival and movements of German agents was made available to British intelligence.
The surveillance operation on the B&B in Liverpool was feared to have been compromised during a botched arrest in 1943 so rather than arrest the owner she was strangled in a staged robbery. Most of the German agents were turned with little encouragement given their limited options, some of those that refused are buried in unmarked graves in the former cemetary opposite Walton Prison in Liverpool
All of the above is from memory so I do stand to be corrected.