By James Davey and Kate Holton LONDON (Reuters) - Britain's biggest retailer, Tesco, wrote down the value of its global operations by $3.5 billion and announced plans to exit the United States, as it sought to rebuild after a year in which profit fell for the first time in two decades. The group, the world's third largest retailer after Wal-Mart and Carrefour, said on Wednesday abandoning loss-making Fresh & Easy in the U.S. would mean restructuring and other one-off costs of 1 billion pounds ($1.5 billion). ...