Preheat the oven to 170°C (fan) or 338°F.
In a small glass or mug, mix 1 tbsp of ground flax seeds with 3 tbsp of water. Stir well and set aside for 3–5 minutes to thicken.
Using a sieve, sift the flour, baking powder, and baking soda into a large bowl. Mix well.
In a separate bowl, combine the rice milk, sunflower oil, maple syrup, flax egg, and the zest and juice of 2 lemons. Mix well.
Make a well in the centre of the dry ingredients, pour in the wet mixture, and stir until a smooth, thick batter forms.
Line a 2lb loaf tin with baking parchment. Pour the cake batter into the tin, spreading it evenly.
Bake the cake in the preheated oven for 40 minutes. If needed, bake for an additional 5–10 minutes, or until a skewer inserted into the centre comes out almost dry. Avoid opening the oven door during baking, as this can cause the loaf to deflate.
While the cake is baking, sift the icing sugar into a bowl. Add the juice of 1 lemon and whisk until the sugar dissolves. For a thicker glaze, add more icing sugar. For a thinner glaze, add more lemon juice or water (1 tbsp at a time).
Once the cake is baked and out of the oven, drizzle the glaze over the cake while it is still warm. Allow the cake to cool fully.
Leave the cake to cool completely before slicing (if you can resist!).