Cinema Management Group is handling international pre-sales.
McElhone will play Dr James Miranda Barry, Britain's first female doctor who was forced to disguise herself as a man in order to practice medicine. Set in 1825 in the Cape, the film tells of the secret love affair between Barry and Lord Charles Somerset, played by James Purefoy.
McElhone stars opposite David Duchovny in the Showtime series Californication and recently wrapped filming on the family fantasy adventure The Secret Of Moonacre. Purefoy played Mark Anthony in HBO's Rome and will star in Barry Levinson's remake of The Saint.
Focus Films' David Pupkewitz and Malcolm Kohll are producing and Peter Dale serves as executive producer.
CMG's Cannes slate includes documentary The People Speak and Focus Films' thriller Chemical Wedding with Simon Callow.