Heat oven to 220C/200C fan/gas 7. On the coarse side of a box grater, grate the butter into a bowl, then place back in the freezer for 5 mins. With a spatula, quickly mix the butter with the flour, baking powder and sugar, add the milk and lemon juice, then bring together until you have a butter-speckled dough. Knead the dough a few times on a floured surface. Roll out to a circle, press into a lightly greased 20cm sandwich tin, then brush the top with egg and scatter with a little sugar. Bake for 20 mins until risen and golden, then transfer to a cooling rack.
While the scone is cooling, toss the strawberries in a bowl with a sprinkling of the sugar and the jam, then set aside. In a separate bowl, whisk the clotted cream until stiff with the 50g sugar and a drop of vanilla extract.
To assemble the cake, place the scone base on a serving plate, spread the cream over, leaving a slight edge of the cake showing, then pile the saucy strawberries on top. Serve cut into wedges