Prepare the Oven and Tin: Preheat the oven to 180°C (350°F), 160°C for a fan-assisted oven, or Gas Mark 4. Line a 900 g (2 lb) loaf tin with greaseproof paper and set it aside.
Combine the Cake Ingredients: Sift the self-raising flour into a large mixing bowl over the room-temperature margarine or butter, caster sugar, eggs, milk, and lemon zest. Mix thoroughly with a wooden spoon until all the ingredients are incorporated into a smooth, evenly combined batter. Fill the Loaf Tin: Transfer the prepared batter into the lined loaf tin, spreading it evenly in the tin.
Bake the Cake: Place the tin in the preheated oven and bake for 45–50 minutes, until the top is golden and a skewer inserted into the cake comes out clean.
Rest and Pierce the Cake: Remove the cake from the oven and leave it in the tin for 5 minutes. While it is still warm, use a skewer to make holes across the top so the lemon syrup can soak into the cake.
Prepare the Lemon Syrup: In a small bowl, combine half of the icing sugar with the freshly squeezed juice of the lemon. Stir until the mixture forms a thin, syrup-like icing.
Soak the Cake: Spoon half of the prepared lemon syrup evenly over the warm cake, allowing it to seep through the holes and into the crumb.
Make the Finishing Glaze: Add the remaining icing sugar to the leftover lemon syrup and mix until it forms a thin icing suitable for drizzling over the cake.
Glaze the Cake: Spoon the finishing icing over the top of the cake while it is still warm, creating a delicate lemon glaze across the surface.
Add the Optional Garnish: If desired, sprinkle a little extra lemon zest over the glaze for a fresh citrus finish. Allow the cake to cool before slicing and serving.