Put the flour, baking powder and sugar in a large bowl and mix together. Gradually pour in the double cream and lemonade, stirring with a wooden spoon, until the mixture comes together into a dough. Tip the dough out onto a lightly floured surface and knead until smooth – be careful not to overwork it.
Transfer the dough to a sheet of lightly floured baking parchment, then roll out to a 2cm thickness. Chill for 1 hr until slightly firm. Heat the oven to 220C/200C fan/gas 7. Take the dough out of the fridge and use a 6cm round cutter to stamp out 10 rounds. Try not to twist the cutter as you press it into the dough.
Line a large baking tray with baking parchment, and arrange the scones on top, leaving space between each. Flip them over so the side that was touching the baking parchment when stamping them out is now on top. Carefully brush the tops with the beaten egg, making sure the glaze doesn't drip down the sides.
Bake the scones for 12-14 mins, or until golden. Leave to cool on a wire rack, then serve.