It's important that the butter is cold when you add it to the dry ingredients. The little flecks of butter in the dough create the rise in the scones as they bake. If the butter is too warm, it will incorporate into the dough and you'll lose that rising power.
To make an egg wash, beat 1 egg and 1 tablespoon water with whisk. Brush onto tops of scones before baking.
For a cleaner look, you can also use a piping bag to create the cross on the scones.