Buns: In a large bowl mix the sugar, soy milk and yeast together. Let sit until yeast froths (about 3 minutes).
In a separate small bowl, mix flax seed and water together. Let sit for 1 minute to hydrate then whisk until thickened. Whisk in orange juice and add to the yeast mixture.
Add 3 1/2 cups of flour, mixed spice, margarine, zests and salt then form into a dough. Add more liquid or flour if needed. The dough should be firm but still tacky. Knead on a lightly floured surface for 5-8 minutes until smooth. Knead in the sultanas right at the end.
Place in an oiled bowl then turn the dough to coat. Cover and let it rise until doubled (1 to 1 1/2 hours).
Line a tray with baking paper. Divide the dough into 12 equal pieces and shape each into a ball. Place on the tray and press down to flatten. Mist with oil, cover and let rise until about doubled (30-45 minutes). Preheat oven to 200 degrees C.
Before baking, uncover and cut a shallow cross into the top of each bun with a razor blade or sharp knife. Brush with soy milk and bake for 15-17 minutes until a deep golden brown.
Glaze: While buns are baking bring glaze ingredients to bubbling simmer over medium heat in a small saucepan. When buns come out of the oven brush with the glaze. Let sit for 1 minute then brush with glaze again. Let cool on the tray.
Icing: When the buns are cool mix together icing ingredients to a firm consistency then fill in the cross shapes with a piping bag.