Line a baking sheet with parchment paper.
Sift together the flour and baking powder in a large bowl, then stir in the sugar and salt.
Add the cubed, chilled butter and rub into the dry ingredients using a pastry cutter, the back of a fork, a food process or stand mixer, or simply your fingers (my preferred approach). Stop once you've reached a coarse, crumbly stage, where the butter resembles small pebbles in the sandy dry dough.
Make a well in the center of the bowl, then slowly add the cream and milk (and poppy seeds and lemon peel if using). Stir together gently with a wooden spoon (or your hands) until just incorporated.
Turn dough out onto a lightly floured surface and press into a block. Let rest 5 minutes.
After resting, fold the dough in half, then lift and turn the dough over onto itself. Press together and let rest 5 minutes.
Roll the dough out to 1-inch (2.5 cm) thickness. then cut into even rounds with a 2 ½-inch biscuit cutter. You will need to reshape the dough a second time to use the scrapes from the first cutting, but try to not overhandle the dough.
Place formed scones on lined baking sheet, cover with plastic wrap, and freeze for 10-20 minutes to rest and help scones keep their shape during baking.*
Preheat oven to 390 F.
Remove scones from freezer after resting, brush with egg wash and bake for about 18-22 minutes. The scones should be just golden brown around the edges.
Remove from oven and let cool on tray for 5-10 minutes. Transfer to wire rack and let cool completely (or eat while warm). Serve with butter, jam, and/or clotted cream.