Soak the Fruit: Place the raisins, sultanas, dried black currants, candied lemon peel, and candied orange peel in a bowl. Pour the hot, very strong black tea over the fruit, stir well, cover, and leave to soak overnight.
Reserve the Fruit Liquid: Check the soaked fruit the next day. It should have absorbed nearly all of the tea, depending on how dry or moist the fruit was initially. If any liquid remains, reserve about 2–3 tablespoons to brush over the finished loaf. Preheat the Oven: Preheat the oven to 325°F and grease a 9 × 5-inch (2-pound) loaf tin.
Combine the Dry Ingredients: Add the all-purpose flour, baking powder, salt, and mixed spice to the bowl of a stand mixer. Stir until the dry ingredients are evenly combined.
Add the Remaining Ingredients: Add the soaked fruit and its remaining juices, excluding the 2–3 tablespoons reserved for finishing, along with the packed dark brown sugar, softened butter, and lightly beaten egg.
Mix the Batter: Fit the stand mixer with the paddle attachment and mix until the ingredients form a batter with the consistency of soft cookie dough. If the mixture appears too dry after mixing, add either a little melted butter or a small amount of egg yolk, as needed. Fill the Loaf Tin: Transfer the batter to the prepared loaf tin. Use the back of a spoon or your fingers to gently smooth the surface into an even layer.
Bake the Loaf: Bake at 325°F for 70–85 minutes, or until a toothpick inserted into the center comes out clean.
Brush with Fruit Liquid: While the loaf is still hot, spoon or brush the reserved fruit-soaking liquid over the top to add moisture and flavor.
Cool the Loaf: Allow the loaf to cool in the tin for a few minutes, then carefully remove it and transfer it to a wire rack to cool completely.
Slice and Serve: Once completely cooled, cut the Bara Brith into slices and serve spread with butter. Traditionally, enjoy it alongside a cup of tea.