The Cloud Walker — Edmund Cooper

The Civilizations of the First and Second Man have been destroyed by the products of their own technology. Now the world is emerging from a new dark age into the dawn of a second Middle Ages. Britain is dominated by a Luddite Church and by the doctrine that all machines are evil.
Into this strange world comes Kieron, an artist's apprentice who is inflamed by a forbidden dream - to construct a flying machine which will enable man to soar through the air like a bird.