Preheat the oven to 170°C (325°F/Gas 3). To make the biscuits, sift the flour, salt, bicarbonate of soda and spices together into a large bowl.
Put the butter, sugar and golden syrup in a large bowl and mix with hand-held electric beaters on medium speed until pale and creamy. Add the egg and egg yolks in a slow stream and mix until well combined. Add the dry ingredients, in three batches, until thoroughly mixed through. Divide the dough into four even-sized portions and flatten each portion into a disc. Cover in plastic wrap and refrigerate for at least 20 minutes, or for up to 3 days.
Remove from the refrigerator and allow the dough to soften slightly. Roll out each disc between two sheets of baking paper until about 3 mm (⅛ inch) thick. Cut into the desired shapes using biscuit cutters or a knife. Re-roll to make use of all the dough.
Place the biscuits on baking trays lined with baking paper and bake, in batches, for 15–20 minutes, or until slightly puffed and golden. Allow to cool on the trays.
Meanwhile, make the royal icing. Sift the icing sugar through a fine sieve. Place the egg white in a bowl and add 1 tablespoon of the icing sugar. Using a wooden spoon, beat to form a smooth paste, then keep adding the sugar, 1 tablespoon at a time, beating well after each addition. Stir in the lemon juice — you should be able to squeeze the mixture through a piping (icing) bag. The icing can be divided into smaller batches and a few drops of food colouring can be added if required. Place the icing in a piping (icing) bag fitted with a fine nozzle and pipe the desired shapes on the biscuits. The biscuits can be eaten when the icing hardens, or stored in an airtight container for up to 2 weeks.