You can mix and knead the dough by hand or machine. As usual, I use my food processor and then finish the kneading by hand. Put the 2 1/2 cups of the flour into a large bowl. Add the salt to one side and the yeast to the other side.
Warm the milk and butter in a small saucepan until the butter is melted and the mixture is lukewarm. If the mixture is hot, let it cool till lukewarm. Pour this into the bowl. Add the sugar and lemon zest and knead the dough, adding as much flour as necessary a little at a time. The dough will be a bit sticky. Knead further by hand until the dough is smooth and elastic and just short of sticky.
Place the dough into an oiled bowl, covered loosely, to rise hour until doubled in size. This should take about an hour and a half or so.
Turn the dough out onto a lightly floured work surface. Lightly deflate the dough and roll it out into a rectangle about 20cm in x 30cm.
Spread the soft butter all over the surface, uniformly. Mix together the sugar, powdered cinnamon and allspice and sprinkle it uniformly over the butter to cover the surface. Sprinkle the currants and golden raisins evenly over this.
Starting from the long side closest to you, roll tightly towards the other side in Swiss roll or jelly roll fashion. Seal the seam well, with a little water if needed, so it doesn’t open up while baking. Trim the ends to neaten.
Cut the roll, with a sharp knife, into 9 pieces of equal width. Arrange the rolls s on a well greased or parchment lined tray (21cm x 21cm or 8-inch x 8-inch) at equal distance from each other.
Cover loosely and allow the shaped dough to rise for about 30 to 40 minutes. They should have puffed out and will be touching each other on the sides.
Bake them at 180C (350F) for about 25 minutes till golden brown. Check after about 15 minutes. If they’re getting too brown, cover them with foil and then finish baking them. Remove the buns from the oven and let them cool slightly before transferring them from the tin to a cooling rack.
In the meanwhile, mix together the icing sugar and water to make an icing that’s thick and of pouring consistency. Once the Chelsea Buns have cooled, drizzle the icing over them. Serve once the icing has set.