Grease a 1 kg (2 lb) loaf tin lightly with butter and line the base with nonstick baking paper. Sift the flour, baking powder and mixed spice into a large bowl and add the courgettes and sugar. Stir well.
Beat the egg and milk together in a jug. Melt the butter in a small pan, then add the raisins and stir well over a gentle heat for a few seconds to help plump them up. Pour the melted butter and milk and egg mixture into the dry ingredients and stir until well combined. Add the walnuts and stir again. Transfer to the prepared tin and level.
Make the streusel topping: mix the flour with the sugar and mixed spice, then rub the butter into the dry ingredients until the mixture resembles fine breadcrumbs. Stir in the walnuts, then scatter over the cake.
Bake the loaf in a preheated oven, 180°C (350°F), Gas Mark 4, for 1 hour to 1 hour 10 minutes until well risen and firm to the touch and a skewer inserted comes out clean. Allow it to cool for 10 minutes in the tin before turning out on to a wire rack to cool completely.