Preheat oven to 200 degrees C. Line two baking trays with baking paper.
Sift flours and mixed spice into a bowl; stir in sugar. Using your fingertips, rub butter into flour mixture until mixture resembles breadcrumbs; stir in sultanas, currants and mixed peel and make a well in the centre. Combine egg and milk in a jug; add egg mixture to flour. Using a flat-bladed knife, mix to form a sticky dough, adding more milk if required.
Spoon rounded tablespoons of mixture, 5cm apart, onto prepared trays. Bake for 15 to 20 minutes or until golden. Stand for 5 minutes in tray before transferring the rock cakes to a wire rack to cool.