Line an 8-cup mixing bowl or pudding basin with two layers of cling film. Cut the crusts off the bread. Place one slice of bread in the bottom of the bowl. Set aside 4 or 5 slices of bread for the top. Line the rest of the slices around the inside of the bowl, overlapping as needed.
In a saucepan, combine the blueberries, blackberries, and strawberries. Add 1 cup sugar and the water. Place over medium heat and cook until the berries begin to release their juices. Add the raspberries partway through cooking to prevent them breaking down too much. Simmer the berries for a couple of minutes until they are juicy but still retain their shape. Add the crème de cassis and lemon juice and remove from the heat.
Use a ladle to spoon the berries along with all the juice into the bread-lined bowl. Arrange the remaining bread slices on top of the berries. Cover the bowl with cling film. Place a plate on top of the pudding and top with a couple of heavy cans. Refrigerate overnight to set.
Fill two empty tea bags with the meadowsweet flowers. Alternatively, use several layers of cheesecloth to tie the flowers up into bundle. In a saucepan over medium heat, warm the heavy cream. Heat the cream just until hot but before it comes to a simmer. Add the tea bags or bundle and allow to cool.
Transfer the cream along with the tea bags to a container. Cover and chill overnight.
To serve, remove the tea bags from the cream. Transfer the infused cream to a mixer and beat with a whip attachment. Add the remaining 2 tbsp sugar and whip the cream until it forms soft peaks. Flip the summer pudding onto a serving plate and remove the bowl. Peel the cling film off the pudding and top with some of the meadowsweet cream and fresh berries. Cut the pudding into wedges and serve with extra meadowsweet cream. Enjoy!