“No one enters Slough House by the front door,” the novelist Mick Herron writes in Dead Lions, the second book in his series about an “administrative oubliette” for useless spies exiled by MI5, Britain’s domestic-intelligence agency. “Instead, via a shabby alleyway, its inmates let themselves into a grubby yard with mildewed walls, and through a door that requires a sharp kick most mornings, when damp or cold or heat have warped it.” The rest of Slough House isn’t much better: a nest of abandoned keyboards and empty pizza boxes strewn around by agents who would rather be anywhere else. On the top floor is the lair of the spymaster Jackson Lamb, stinking of “takeaway food, illicit cigarettes, day-old farts and stale beer.” Herron’s spy-novel series is now 13 years old, the same age as Britain’s floundering Conservative government. After years of obscurity, his books are now best sellers, and Apple has so far adapted three for television under the name Slow Horses, after the first novel in the series. The reviews of the show’s newest …