Preheat the oven to 400°F (200°C).
In a large mixing bowl, whisk together flour, sugar, baking powder, and salt.
Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
In a separate bowl, combine buttermilk and the egg, whisking until combined.
Pour the wet ingredients into the dry mixture, stirring gently until just combined.
If using, fold in the raisins or currants gently into the dough.
Turn the dough out onto a lightly floured surface and pat it into a circle about 1-inch thick.
Cut the circle into wedges or use a round cutter for individual scones.
Transfer the scones onto a lined baking sheet, ensuring they are spaced apart.
Brush the tops with a little buttermilk or egg wash for a glossy finish.
Bake for 15-20 minutes, or until golden brown and risen.
Remove from the oven and allow to cool on a wire rack before serving.