Combine first 4 ingredients in a large bowl; cut in margarine with a pastry blender until mixture resembles coarse meal. Add buttermilk, stirring with a fork just until dry ingredients are moistened. Stirin raisins.
Sprinkle 1 tablespoon flour evenly over work surface. Turn dough out onto floured surface, and knead 4 or 5 times. Pat dough to 3/4-inch thickness; cut into rounds with a 2-inch biscuit cutter. Place rounds on a baking sheet coated with cooking spray. Brush tops of rounds with skim milk. Bake at 425° for 11 minutes or until golden. Serve immediately.