Preheat the oven 160C/325F/Gas 3. Line a baking tray with greaseproof paper.
Cream the butter and sugar together in a bowl until light and fluffy. Beat in the egg yolk until well combined.
Fold the flour into the mixture, then stir in the mixed spice, currants and candied peel. Stir in enough milk to form a stiff dough.
Roll the dough out onto a floured surface and cut out the biscuits with a fluted cutter. Place onto the baking sheet and bake for 10 minutes.
Remove the biscuits from the oven, brush with the reserved egg white, sprinkle with sugar and return to the oven for 5-10 minutes, or until pale golden-brown.
Remove the biscuits from the tray and set aside to cool on a wire rack.
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