In the spy series “Slow Horses” on Apple TV+, Slough House is the dumpy, narrow building somewhere in London that serves as headquarters for MI5′s castoffs. Rather than the U.K.’s best and brightest, these are the slow horses of the title — stuck on an island of misfit toys, but for intelligence officers — and this is where young River Cartwright (Jack Lowden) finds himself after colossally botching a training exercise. Pensive, headstrong and ansty, the last thing he wants is to spend his days digging through the trash of a hack journalist they’re surveilling. But that’s exactly what his boss…
