Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8x12 inch pan. Sift together the flour, baking powder, 1 teaspoon mixed spice and 1/2 teaspoon nutmeg. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour mixture alternately with the milk. Spread batter into prepared pan. Sprinkle the chopped rhubarb over the batter. In a small bowl, mix together the brown sugar, 1/2 teaspoon mixed spice, 1/4 teaspoon nutmeg and slivered almonds. Sprinkle over top.
Bake in the preheated oven for 50 to 60 minutes, or until the cake springs back when lightly tapped. Serve warm or cold.