Preheat the oven to 180°C, 350°F, Gas Mark 4.
Place the flour, condensed milk, egg, vanilla extract, margarine and baking powder in a bowl and beat with an electric hand mixer for 2-3 minutes until pale and fluffy.
Spoon into the tins and bake for approximately 25-30 minutes until springy to the touch and golden brown. Cool slightly then transfer to a cooling rack to cool completely.
Place one half of the cake onto a serving plate and spread with the jam. Sandwich together with the other half.
Beat the icing sugar, butter and vanilla with a few drops of water to bring the mixture to a soft consistency. Spread onto the cake. Arrange the berries on top in a Union Jack shape if you like!