LONDON — British exhib ABC Cinemas, the second biggest in the country, is planning to sell 20 of its city center sites, a move forced by competition from new multiplexes.Ray Wallis, ABC marketing director, said a formal review had determined that the sites were no longer “economically viable” because their admissions had collapsed almost immediately after the opening of out-of-town multiplexes. The sell-off of the single, two- or three-screen cinemas will cost 200 jobs and leave ABC with 60 sites, including its only multi, an eight-screener, which recently opened in Edinburgh. The exhib is bowing two more multis in the small northern England cities of Mansfield in December and Rochdale in May 1998. Three other plexes are planned. ABC is an old British cinema brand that was resurrected in May last year when former ABC managing director Barry Jenkins engineered the acquisition of 90 cinemas from Richard Branson’s Virgin Group for $109 million. Virgin’s exhib division retained all of its multis in the deal, and most of the cinemas it sold to ABC had once been part of the original ABC chain.