Preheat the oven: Preheat to 160°C and line a baking tray with nonstick baking paper.
Prepare the dry ingredients: In a mixing bowl, whisk together the flour, sugar, and baking powder. Add the cubed butter and rub it into the flour mixture until it resembles coarse breadcrumbs. Mix in the lemon zest.
Prepare the wet ingredients: Measure the milk and squeeze the lemon juice into it to make homemade buttermilk. Whisk in the egg until well combined.
Combine the ingredients: Pour the buttermilk mixture into the flour mixture, add the blueberries, and gently mix until the flour is fully incorporated. The dough will be sticky.
Shape the scones: Turn the dough onto a floured surface and shape it into a flat, round disc about 1.5 cm thick. Dust your hands and the dough with flour as needed. Cut the disc into 6 scones.
Prepare for baking: Place the scones on the prepared baking tray. Brush the tops with a little milk and sprinkle with demerara sugar.
Bake: Bake in the preheated oven for 20-25 minutes, or until the scones are golden brown. Allow them to cool on a wire rack.
Make the lemon glaze: In a small bowl, mix the icing sugar and lemon juice until smooth. Drizzle the glaze over the cooled scones.