Preheat the oven to 425°F (220°C). Place a baking sheet in the oven while it heats.
Add the flour, baking powder and salt to a large bowl and stir to combine. Add the cold butter and rub it into the flour with your fingertips until the mixture resembles fine crumbs.
Stir in the sugar. Mix the milk (gradually - you might not need every drop), lemon juice and vanilla extract in a pitcher.
Pour most of the milk mixture into the flour. Stir gently until the dough begins to come together, adding the remaining milk only if needed. Fold in the raisins, sultanas or currants, if using.
Transfer the dough to a lightly floured surface. Bring it together gently without kneading it heavily. Pat the dough to about 1 ½ inches thick.
Cut out rounds with a floured cutter, pressing straight down without twisting. Gently bring the scraps together and cut out the remaining scones.
Carefully remove the hot baking sheet from the oven. Arrange the scones on it and brush only the tops (not the sides) with beaten egg.
Bake for about 10 minutes, or until well risen and golden. Transfer the scones to a wire rack and allow them to cool slightly before serving with strawberry or raspberry jam and clotted cream.
