Johnny Flynn is a South African-born, U.K.-raised singer-songwriter in his mid-20s who makes a British folk rock that sounds almost twice as old as he is. The buoyant, spindly blend of fiddles, fingerpicking, soft-brass fanfares and pub-choir harmonies on Flynn’s marvelous American debut, A Larum (Lost Highway), made with his band the Sussex Wit, is a direct descendant of the pot-smoke, minstrelsy, real-ale purism and gently electric modernism of the early-Seventies Incredible String Band and Fairport Convention. Flynn’s voice is a similar mix of then-in-now, with flashes of a less-weathered Bert Jansch and a cocky, Celtic Beck. That would all be enchanting mimicry if Flynn was a lesser songwriter. But Flynn – who toured as a young Shakespearean actor before he started singing for his supper – knows how to draw and inhabit a scene: the humor and flinty pride amid the detailed poverty in “The Box” and “Leftovers”; the mocking contrast of jaunty rhythm and imprisoned spirit in “Tickle Me Pink”; the passion and challenge Flynn packs into the stark title image and chorus of “Cold Bread.” Flynn’s lyrics sometimes strain their settings. But even when his images and exchanges don’t quite jell into stories, the jigs and jangle are natural magic. As a first, giant step, A Larum (the title is Old English for “alarm”) is a dramatic entrance.