Preheat the oven to 180ºC, fan 160ºC, gas 4. Grease and line a 30cm x 20cm Swiss roll tin. Put the butter and sugar in a large bowl and, using an electric hand whisk, beat until fluffy. Beat in the eggs until combined.
In a separate bowl, combine the flour and ground almonds and then gently fold into the egg mixture until just combined. Stir in the milk. Spoon the batter into the lined cake tin. Bake for 20 minutes until a skewer inserted into the centre comes out clean. Leave to cool in the tin on a wire rack.
Divide the icing in half. Take one piece and knead in enough blue food colouring to get a deep shade of blue – do this gradually to ensure an even colour.
Divide the remaining icing into 2 pieces. Take 1 piece and knead in enough red food colouring to get a deep shade of red – do this gradually to ensure an even colour.
Brush the cake all over with the strawberry jam. Roll out the final piece of icing to 30cm x 20cm and place on top of the cake – drape it over the rolling pin to help position it. Cut the cake into 12 rectangles.
Roll out the red and blue icing and cut into strips and triangles to make into Union Jack flags. Dampen the underside of each of the pieces and stick to the white background. Use the strawberry laces and the decorating balls to finish.