Keira Knightley says it’s exciting to be opening the London Film Festival with World War II code-breaking story “The Imitation Game.” The film stars Benedict Cumberbatch as computer pioneer Alan Turing, who helped win the war but was later prosecuted for homosexuality. Knightley, who plays fellow code-breaker Joan Clarke, said Wednesday that “it’s lovely to be the first” of the 248 features screened at the 12-day festival.