Cut the cake into small cubes and layer them evenly on the bottom of your trifle dish. Drizzle with orange liqueur or juice so the sponge soaks up just enough flavor without becoming soggy.
Heat half the cranberry juice until just simmering. Sprinkle gelatin over the remaining room-temperature juice and whisk gently. Pour the hot juice into it to dissolve the gelatin completely. Pour half of this mixture gently over the cake layer and refrigerate for 1½ hours until semi-set.
While the first jelly layer sets, make your custard. Heat milk, sugar, and vanilla until steaming. In a bowl, whisk egg yolks and cornflour, then slowly add the hot milk mixture while whisking. Return it to the saucepan and cook over low heat, stirring until thickened. Cool slightly with plastic wrap on the surface.
Once the first jelly is set, gently spoon the custard on top, spreading evenly to seal the edges. Refrigerate again for about 1 hour so it firms up slightly before adding the next layer.
Whisk the remaining jelly mixture until slightly thickened but still pourable. Spoon it carefully over the custard. Add half the strawberries, raspberries, and blueberries on top, then chill for 3–4 hours (or overnight) until fully set.
Whip the cream with sugar and vanilla until soft peaks form. Spread the whipped cream evenly over the top. Garnish with the remaining fresh berries and a dusting of powdered sugar just before serving.