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MARGARET THATCHER has long been reviled by the British left, so much so that the singer Elvis Costello once fantasized about stomping on her grave in his 1989 song “Tramp the Dirt Down.” But Mrs. Thatcher achieved more than any other British peacetime prime minister of the 20th century. It is rumored that, when she dies, she will receive a state funeral — an honor rarely accorded to anyone except monarchs. There are also plans for a public celebration.

Her life is the inspiration for a new movie that opens later this month, starring Meryl Streep as “The Iron Lady.” It chronicles Mrs. Thatcher’s divisive policies as prime minister as she led Britain through the economic doldrums of the 1980s. It was a time when the country faced financial ruin...