Sift the flour, custard powder and icing sugar into a bowl. Rub the butter using your fingertips into the flour until the mixture resembles fine breadcrumbs. Sprinkle over the measured water, then using a round-bladed knife begin to work the mixture into a smooth, firm dough. Wrap and chill for 15 minutes.
Toss the blackberries and apples with the sugar and honey and place in a 20 cm (8 inch) round pie dish or ovenproof dish.
Roll out the pastry, on a well-floured surface, to a round slightly larger than the pie dish. Place the dish on the pastry and cut around it using a sharp knife to produce a circle the correct size for the top. Using the pastry trimmings, make a border strip about 1 cm (½ inch) wide. Dampen the edges of the pie dish and fit the strips of pastry around the edge, pressing firmly. Dampen this pastry too before placing the circle on top and pressing again firmly to hold in place. Using any remaining trimmings, decorate the pie with leaves, flowers, birds or other shapes.
Brush with beaten egg to glaze and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until golden and crisp. Serve with vanilla ice cream or crème fraîche, if liked.