LONDON, May 21 (Reuters) - Nationwide Building Society reported on Friday that its annual profits nearly doubled, as it improved interest income and margin while shaving costs. The bellwether mortgage lender reported pretax profit for the year to April 4 of 823 million pounds ($1.17 billion), compared to 466 million pounds the previous year. But Nationwide added its member financial benefit, which is the value it delivers to its member-customers through beneficial rates and other measures, was below a 400 million pound target. ($1 = 0.7045 pounds) (Reporting by Lawrence White and Iain Withers Editing by Rachel Armstrong) Our Standards: The Thomson Reuters Trust Principles.