Preheat oven to 170 C / Gas 3. Grease and flour a 26cm tube cake tin. Mix together the flour and baking powder.
In a large bowl, cream together 225g butter, 100g margarine and 400g sugar until light and fluffy. Beat in the eggs, one at a time. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Stir in 2 tablespoons rum, key lime zest, 2 teaspoons key lime juice, vanilla extract and lemon juice.
Pour batter into prepared tin. Bake in the preheated oven for 90 minutes or until a skewer inserted into the centre of the cake comes out clean. Allow to cool in tin 10 minutes. Turn it out onto wire rack.
Meanwhile, make the glaze. In small saucepan, combine 4 tablespoons sugar, 50g butter and 2 tablespoons key lime juice. Bring to the boil. Continue to boil, stirring constantly, for 1 minute. Remove from heat and stir in 3 tablespoons rum.
While cake is still warm, prick the top with a skewer. Pour glaze all over warm cake. Cool completely.
Are available at some speciality markets. If you can't get your hands on any, use regular limes.