Preheat the oven to 425°F (220°C). Have ready an ungreased large sheet pan.
Put the flour, sugar, and baking powder into a large bowl and stir together. Scatter the shortening and butter cubes over the top and, using a pastry blender or your fingers, work in the butter and shortening until the mixture is the consistency of bread crumbs. Whisk the egg in a small bowl until blended, then whisk in the milk. Add the milk mixture to the flour mixture and mix until a slightly crumbly but cohesive dough forms. Add the currants, if using, and gently knead into the dough.
Divide the dough into 16 equal portions weighing about 2 oz/60 g each and shape each portion into a ball. Arrange the balls on the sheet pan, spacing them well apart, and press down lightly on each ball to flatten the top. Make two cuts almost all the way through at right angles to each other, so that you form four wedges, which can easily be torn apart when served. (Alternatively, on a floured work surface, roll out the dough to about 1½ inches (4 cm) thick and use a 2-inch (5-cm) round biscuit cutter to cut into rounds.) Brush the tops with milk.
Bake until golden brown, about 15 minutes. After 10 minutes, it is worth turning them over to brown the bottoms, if you like. Transfer to a wire rack and let cool before serving.