Preheat the oven to 300 degrees F. Grease and line a 5 by 9-inch loaf pan with greaseproof or parchment paper, and set aside.
In a large bowl, beat the eggs, and then add the 5 ounces oil, brown sugar, grated carrot, raisins, and chopped nuts, if using.
Sift in the flour, salt, baking soda, cinnamon, nutmeg, and mixed spices, and bring the mixture together using a wooden or large metal spoon.
Pour the mixture into the prepared loaf pan, smooth the surface, and bake in the oven for 1 to 1 hour 15 minutes, or until a skewer inserted into the middle comes out clean.
Allow the cake to cool in the pan for about 5 minutes before removing. Cool the cake completely on a wire rack before serving.
To make the icing: In a bowl, beat the cream cheese and butter together, until combined. Add the vanilla extract, confectioners' sugar, and finely grated orange zest, and mix to combine. The icing should be smooth, and quite thick. Using a palette knife, spread the icing evenly over the cooled cake, dipping the knife into a bowl of hot water if the icing is hard to spread out. Cut the cake into slices to serve.
Uniced, this cake is also delicious sliced and buttered.