Preheat your oven to 180°C (160°C) and line a 9inch cake tin with baking paper.
In a bowl, mix together the flour, baking powder and salt, set aside.
In a large mixing bowl, combine the oil, sugar, egg, vanilla extract and 10ml of milk. Whisk very well for a few minutes using a hand mixer.
Gradually add the flour mixture, alternating with the milk, little by little, whisking in between each addition until smooth.
Transfer your cake mixture to your lined cake tin.
Arrange your strawberries and blueberries on top to form the Union Jack flag shape, covering the whole of the cake.
Sprinkle 1 ½ tbsp of caster sugar over the top of the strawberries and blueberries.
Bake for 10 minutes, then lower the oven temperature to 160°C and bake for 50 minutes – 1 hour, or until a skewer comes out clean.
Leave in the cake tin on a wire rack until completely cool, then tip out and serve.
Store in an airtight container in the fridge and enjoy at room temperature, within 3 days.