Preheat an oven to 180 C / Gas 4. In a small bowl, combine digestive biscuit crumbs, sugar and melted butter. Press mixture evenly into the bottom of a cake tin. Bake for 8 minutes, and remove from oven. When cool, break into small pieces.
Combine buttermilk, condensed sweetened milk, cream cheese, lemon juice, zest and salt in a blender or food processor. Process until smooth. Pour into an ice cream maker, and freeze according to the manufacturer's instructions.
Mix in cashews and pieces of digestive biscuit base. Gently fold in preserves to create a ribbon effect. Transfer to the freezer for 1 hour to harden.