Boil 1 cup water and mix with instant coffee in a measuring cup or mug. Set aside to cool.
Preheat the oven to 350 degrees F (175 degrees C). Grease a rectangular 9x13-inch baking pan with margarine.
Combine flour, demarara sugar, mixed spice, baking powder, salt, cinnamon, nutmeg, and ginger in a bowl and mix well. Add cooled coffee, oil, and 3/4 cup water and mix until well combined. Fold in pumpkin puree and vanilla extract; mix well. Pour batter evenly into the prepared baking pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean or almost clean, as this cake can be quite sticky, about 30 minutes.
Cool on a wire rack for 5 minutes. Run a table knife around the edges to loosen. Invert cake carefully onto a serving plate or cooling rack. Cool at least 15 minutes more before cutting.