Pour hot tea over currants and raisins in a bowl; soak 8 hours to overnight. Preheat oven to 350 degrees F (175 degrees C). Grease a sheet of parchment paper and use it to line a 2-pound loaf pan. Stir demerara sugar into tea and fruit mixture until dissolved completely. Mix flour and egg into the tea and fruit mixture until completely integrated into a batter. Spread batter evenly into the prepared bread pan. Bake in preheated oven until golden-brown, about 40 minutes. Cool in the bread pan 5 minutes before transferring to a rack to cool completely.