Madonna showed up for the British premiere of her movie "The Next Best Thing" in London last night and showed off her pregnant belly for eager reporters and photographers. "It's really funny," she announced, "but I've got this craving for eggs and olives." At the moment, she told them, she would have settled for a bag of potato chips. The five-month-along star of the movie, which costars Rupert Everett and fared very poorly with critics and at the box office when it was released in America earlier this year, said she was wearing a black outfit to the London premiere because "it's a slimming color. Anything else makes me look like a sofa."