Preheat oven to 180°C and line two trays with greaseproof paper.
Mix flour, baking powder, mixed spice, and sugar in a bowl.
Stir in orange zest, currants, cranberries, and chopped nuts.
Add beaten eggs and knead by hand into a dough.
Divide dough into four pieces and roll each into 30cm logs.
Bake for 25 to 30 minutes at 180°C until firm but pale.
Reduce oven temperature to 140°C.
Cut dough diagonally into 1cm slices using a bread knife.
Bake slices for 15 minutes until dry and golden-brown.
Whisk sugar and cream until soft peaks form.
Fold in orange zest, juice, and milk until a dip consistency is reached.
Serve biscotti with the syllabub dip.