This is only a partial answer, but my impression from The Authorised History of MI5 is that an important part was ideological appeal, at least when it comes to the success of British counter-intelligence. Nazism simply didn't hold much appeal to the British establishment, educated classes, or much of the population in general. This made the pool of potential agents limited, and often poorly motivated. Thus, when they tried to infiltrate Britain, Abwehr agents tended to get captured and/or defect.

This point is comes out when you contrast with post-war Soviet espionage, which was tremendously successful. Communism did have a strong appeal to some members of the British elite. They were able to get into important positions, such as MI5, and there was reluctance to question their loyalty or take action against them.