An inspirational true story of a group of friends from a working men’s club who decide to take on the elite ‘sport of kings’ and breed themselves a racehorse. Louise Osmond’s “Dark Horse,” an account of impecunious Welsh villagers who team up to sponsor a race horse that surprises everyone by turning out a winner, has all the elements of crowd-pleasing Britflicks such as “The Full Monty,” “Billy Elliott” and “Pride.” It almost cries out to be a Mike Leigh film starring Jim Broadbent and other members of the director’s stock company.