Slough House denizens screw up in blatant, over the top ways. While the Park screw up in ways that leave geopolitical consequences festering for years or decades while being good at covering their own asses.

The plot is generally some evil, corrupt actions the Park took in the past are coming home to roost and only the bumbling losers in Slough House can fix it (kind of, eventually, in a "at least London wasn't blown off the face of the earth" kind of way).