Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking powder, sugar, and salt. Add the butter and work it into the flour mixture with your fingers or a pastry blender, until it resembles a coarse meal. Stir in zest and currants.
In a small bowl, beat the egg and 4 tablespoons of the cream together with a fork. Add to the flour mixture and mix with your hands until the dough just comes together. (If the dough seems dry add the extra tablespoon of cream.) Turn the dough out onto a lightly floured work surface. Pat the dough into a 6-inch round about 1-inch thick. Cut into 8 equal sized wedges. Space the scones evenly on the prepared baking sheet. Bake until golden brown, about 12 to 15 minutes. Serve warm.
For a richer, darker crust, brush the tops of the scones with heavy cream and sprinkle with sugar before baking.