Heat the oven to 180° / 160°C fan. Grease a baking pan (approx. 30 x 25cm) + line with baking paper.
Sift together the flour, baking powder, mixed spice + bicarbonate of soda in a large bowl. In another bowl mix the sugar, oil, soup + water. Make a well in the centre of the dry ingredients + tip in the soup mixture. Beat with an electric mixer on low speed until fully blended. Pour the batter into the pan.
Bake for 40 minutes or until a toothpick inserted in the centre comes out clean. Let the cake cool in the pan on a wire rack for 20 minutes.
Beat the margarine, soya cream cheese + vanilla extract in a medium bowl with an electric mixer until the mixture is creamy. Slowly beat in the sieved icing sugar, adding a little soya milk if needed to soften. Frost the cake with the cream cheese mixture.