In a huge TV studio, a beautiful, slender-limbed girl whirls about lasciviously in a leotard embellished with silk stockings and a garter belt, the embodiment of sexual allure. Off to the side, a rumpled, bohemian-looking gentleman of 53 years watches her fixedly, then notes in a soft British accent, "She has a very beautiful body, perfect proportions for dancing."

As principal choreographer of London's Royal Ballet Company, Sir Kenneth MacMillan knows dancing talent, and he calls Italian ballerina Alessandra Ferri, 20, "one of the most talented young dancers in the world." MacMillan selected the 5'3", 96-lb. Ferri to star in his Granada TV production of Weill/Brecht's Seven Deadly Sins (which may be released in America early in 1984). Besides personifying Lust, Alessandra will be seen leaping the sinful gamut from Gluttony to Envy.

The elfin Ferri vaulted to prominence in 1982 when she danced the lead in MacMillan's Mayerling, winning raves in London and later at the Met in New York. All that, however, meant less to Ferri than the reaction of her parents. The daughter of a housewife and the manager of a car-parts factory in Milan, she says, "My parents didn't want me to pursue ballet" because it was too chancy. After witnessing her London triumph, though, "They were just so excited for me they could not talk."

Ferri's gifts were apparent from almost her first plié. At age 5, she insisted upon attending ballet class in her Catholic school, and the nuns quickly arranged an audition at La Scala. Studying there from age 10 to 15, she won admission in 1979 to the Royal Ballet School. Now a soloist with the Royal Ballet, Alessandra lives alone in a small, recently purchased London flat. The solitude suits her. "There is no time for boyfriends," she declares crisply, content for now to have all her pas de deux onstage.