Preheat the oven to 300°F Grease two 8 x 4-inch loaf pans and line them with parchment paper.
In a large bowl, cream together the butter and sugar. Add the milk and beat until light and fluffy. In another bowl, combine the coconut, nuts, raisins, candied fruits, and lemon zest. Add 1/2 cup of the flour and mix well. In a separate bowl combine the remaining flour with the baking powder and salt. Add the fruits and the flour mixture to the creamed mixture and stir to combine thoroughly. In another bowl, beat the egg whites until stiff. Fold the whites and vanilla into the batter until well blended. Turn the batter into the prepared pans, spreading it evenly.
Bake for 2 hours 15 minutes or until a skewer inserted into the center of the cakes comes out clean. Cool in the pans for 15 minutes. Turn the cakes out of the pans and finish cooling on a wire rack. To keep, wrap in plastic and store for up to 4 weeks. Makes 2 loaf cakes.