Preheat the fan-assisted oven to 190 °C / 375 °F. Line one large baking sheet or two small baking sheets with parchment paper.
Add flour, sugar, lemon zest, baking powder, and vanilla to a large bowl. Grate the cold butter into the mixture. Rub the mixture with your fingers until you get a sand-like texture. In a separate bowl, whisk together the milk and egg. Pour most of the milk mixture (leave about 2 tablespoons) into the dry ingredients mixture and stir to combine with a rubber spatula until just combined. Add in the poppy seeds, and blueberries, then knead with your hands until it comes together. Don't overwork the mixture. Fold the dough a couple of times to create layers.TipYou can use fresh blueberries too.
cut the scones
Transfer the dough to a lightly flour-dusted working surface. Divide the dough into two equal parts, then shape each piece of the dough into a circle, approximately 3.5 cm - 4 cm or 1 1/2 inch thick. Cut the circle in half, then cut each half into three wedges. You should get 12 wedges from both of the circles.TipFeel free to freeze the shaped rectangle scones at this point.
brush the scones
Place the scones on the prepared baking sheet, leaving some space between them to rise. Brush with the remaining milk mixture, and sprinkle with Demerara sugar. Place in the freezer for 15 minutes.TipIf you don't have space in your freezer, place in the fridge for 30 minutes, or leave this part out completely.
Place the lemon blueberry scones in the oven, on the middle rack, and bake for 18 - 20 minutes, or until golden-brown. Rotate your baking sheet halfway during the baking process. Transfer the baked scones to a wire rack to cool, then serve.