Grease a large loaf tin that will fit on the lower rack of the halogen oven. Line the bottom and long sides with a strip of greaseproof paper. Grease the paper.
Put the flour and butter in a food processor. Process until the mixture resembles fine breadcrumbs. Add the sugar, eggs and milk and process to a dough.
Spread half the cake mixture into the loaf tin and scatter with half the apples and blueberries. Spread the remaining cake mixture over the top, then scatter with the remaining fruits. Place on the lower rack of the halogen oven.
Set the temperature to 200°C (392°F) and cook for about 45 minutes, covering with foil if the surface starts to brown too much, until a skewer inserted into the centre comes out clean. Leave in the tin for 10 minutes, then transfer to a plate or board. Sprinke with vanilla sugar and serve warm, in slices, with clotted cream.