Son of the veteran Brit doc-maker Mike Dibb, Saul Dibb has been on the list of young British directors to watch since his early documentary shorts. His feature debut, “Bullet Boy,” smacked a little uncomfortably of white liberal tourism into the fashionable topic of black gun crime, but Dibb’s eye and dramatic talent were plain to see. “The Line of Beauty,” his elegantly crafted miniseries about gilded Thatcherite youth, was widely acclaimed. Now he has stepped into a new dimension with the 18th-century romantic drama “The Duchess,” a glossy Keira Knightley vehicle that could finally establish him on Blighty’s A-list.