LONDON — Colin Firth is being lined up to play the romantic lead opposite Renee Zellweger in Working Title’s “Bridget Jones’ Diary,” with Hugh Grant as his rival.The casting of Firth, who played supporting roles in “The English Patient” and “Shakespeare in Love,” is peculiarly apt. His character, Mark Darcy, is based on Mr. Darcy from Jane Austen’s “Pride and Prejudice,” whom Firth played in a BBC television adaptation. Firth’s smoldering incarnation of Austen’s hero obsessed the Bridget Jones character in Helen Fielding’s original novel. Pic, directed by Sharon Maguire, starts shooting May 1. Jonathan Cavendish is producing alongside Working Title’s Tim Bevan and Eric Fellner. Financing and distribution is split among Universal Pictures, Miramax Films and Canal Plus.