Place a rack or overturned shallow bowl in a large pot and fill with water. Bring to simmer.
Grease a 2-quart heat-proof dish. It needs to have a tight-fitting lid. Set aside. (Pyrex will work well.)
Whisk together the flour, bread crumbs, sugar, baking powder, salt, and spices in a large mixing bowl.
Rub in the cold butter until mixture resembles coarse meal (with some larger pieces of butter remaining).
Toss in raisins and currants.
Pour in milk and fold together until mixture is uniformly moistened. Turn out the mixture into the greased dish and press down the top with a spatula.
Cover the dish with lid, making sure it is tightly sealed. Place it in the pot; the water should come halfway up the sides of the dish. Steam for 3 hours. Add water to replenish as necessary.
Remove pudding from pot. Remove the lid carefully, and invert pudding onto a plate. Serve warm with warm custard as desired.