Preheat the oven to 200C/180C Fan/Gas 6. Line/grease an 8" cake tin.
Make the crumble topping by combining all ingredients in a bowl, mash with a fork until mixture looks a bit like sand. Set aside.
In a large bowl/the bowl of a stand mixer, combine the butter and granulated sugar on a medium speed until entirely combined. For real creaming, this should be about 3-5 minutes with an electric/stand mixer. Add the eggs, one at a time, then add the vanilla.
Add the citrus juice and zest and combine again. The mixture may begin to look a little curdled, due to all the citrus/eggs etc, but stick with it!
Combine the flour with the baking soda and baking powder in a small bowl. Add a third of this and a third of the milk, mix until all combined. Then repeat with remaining flour and milk. The mixture will be fairly smooth and should pour easily.
Toss the blueberries with some flour to prevent them from sinking. Gently fold into the cake batter.
Pour the cake batter into the prepared tin. Sprinkle over the crumble topping and bake for 40-45minutes, until golden brown and a cake tester/toothpick comes out clean. Leave to cool on a wire rack.
Meanwhile, make the glaze by combining all ingredients in a small bowl/jug and drizzle over the cake (if making ahead, wait until serving to pour over). Serve immediately.