Seems like it was a mix of berries and some kind of bread with the crusts removed
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Here's Merrill's: https://food52.com/recipes...
I can't vouch for the recipes, but this article from The Guardian seems pretty comprehensive: http://www.theguardian...
Delia Smith has a great recipe: http://www.deliaonline.com/recipes/cuisine/european/english/english-summer-pudding.html
Summer pudding is one of those desserts that's really easy to play around with and make it to your taste. You're right, it's just white bread soaked in berry juice with lots of berries added, so you can experiment with different berries and sugar/honey etc. to find your favorite. British bread is a lot less sweet than American bread but for a dessert that shouldn't be a problem.