Preheat the oven to 350 degrees F (175 degrees C). Grease an 8x4-inch loaf tin with soft margarine and set aside.

Cream superfine sugar and butter together with a paddle attachment of a free-standing electric mixer on medium-high speed until light and fluffy.

Whisk eggs and vanilla extract together in a separate bowl until just mixed; do not over-whisk. Beat gradually into the butter mixture until well incorporated. Sift in flour and baking powder, bit by bit, folding after each addition. Set batter aside.

Puree raspberries, confectioners' sugar, and lemon juice in a food processor until smooth. Strain through a fine-mesh sieve into a bowl. Separate half of the cake mix into a different bowl and stir the coulis through.

Put alternating dollops of the raspberry and vanilla batters into the prepared loaf tin using a small ice cream scoop. Swirl both sets of dollops together slightly using the blunt end of a skewer.

Bake in the preheated oven until the top of the cake is golden brown and risen and a toothpick inserted into the center comes out clean, about 40 minutes. Let cool in the tin for 5 to 10 minutes. Run a table knife around the edges of the tin and remove the cake, leaving to cool completely on a wire rack.