In a bowl, mix raisins, candied peel, and 1/4 cup rum. Let stand 20 minutes.
Meanwhile, grate peel and squeeze juice from 1 orange and the lemon; reserve. Thinly slice remaining orange, discarding ends; cut slices into half-moons. Arrange slices attractively over bottom and up sides of a buttered 9- by 5-inch loaf pan.
In a bowl, with a mixer on medium speed, beat butter and sugar until well blended. Add eggs, one at a time, beating well after each addition. Add grated orange and lemon peel and 2 tablespoons each orange and lemon juice (reserve remaining for other uses).
Add flour, baking powder, baking soda, and salt and beat on low speed until well blended. Stir in rum-steeped raisins and candied peel. Scrape batter into prepared pan, taking care not to disturb orange slices; smooth top level.
Bake in a 350° oven until a wooden skewer inserted in the center comes out clean, 50 to 60 minutes. Let stand 10 minutes, then run a knife blade gently around edges to release cake; invert onto a wire rack.
In a 1- to 2-quart pan over medium heat, stir marmalade and remaining 2 tablespoons rum until hot. Pour through a fine strainer into a small bowl. Brush glaze all over warm cake. Let cool to room temperature, about 1 hour, then cut into slices to serve.