Reserve a handful of the prettiest whole strawberries and blackberries for topping the trifle. Hull and halve the remaining strawberries, and toss them with the blackberries and sugar in a medium bowl, cover with cling film and set aside until they get juicy, about 1 hour. (If you're in a hurry, set the bowl over a pot of simmering water for about 5 mins to speed this up.)
Whip the cream in a chilled bowl until it holds soft peaks. Fold the elderflower syrup into the cream and keep chilled until ready to assemble the trifle.
Tear or cut the angel food cake or sponge cake into 5cm pieces. Scatter about half the cake on the bottom of a trifle dish or clear glass bowl. Ladle half of the berries and their juices on top of the cake, and then top with half the whipped cream. Repeat with the remaining cake, berries and cream to finish the trifle.
Garnish the top of the trifle with the reserved whole berries and refrigerate for 2 and up to 24 hours. Serve chilled.