Line the base of two loose-bottomed 20cm non-stick sandwich tins with baking parchment. Heat oven to 160C/fan 140C/gas 3.
Put all the cake ingredients (except the blackberries and using just half the milk) in a bowl, then beat until creamy and well mixed. Stir in 100g blackberries, along with the extra milk if you need it to loosen the mixture a little. Divide between the sandwich tins and level the tops.
Quickly whisk the egg whites in a clean bowl, adding the 100g sugar, a spoonful at a time, until really thick and glossy. Lightly spread onto one of the cakes, then sprinkle with the flaked almonds and extra sprinkle of sugar. Bake both cakes for 40 mins, then take the cake without the meringue out of the oven and set aside. Bake the remaining cake for 30 mins more until a skewer inserted into the cake comes out clean. Carefully remove cakes from the tins and cool on a wire rack. When the cakes are cold, peel off the paper.
To serve, spread the cream on the plain cake right up to the edges. Spoon over the bramble jelly, then sandwich with the meringue-topped cake and decorate with the remaining blackberries.