Combine flaxmeal and water in a small bowl; set aside.
Combine flour, sugar, baking powder, and salt in a medium bowl. Cut butter into mixture with a pastry cutter or with fingers until mixture resembles coarse meal.
Add cream and flax slurry to to mixture and stir till just combined. Add grapefruit juice, rind, and kumquats; mix until well combined. Transfer mixture to a baking sheet lined with parchment. Pat dough into a circle, about 1 1/2-inches thick. Freeze dough for 20 minutes.
Cut dough into 8 wedges, leaving room between to spread while baking. Freeze again for 20 minutes.
Preheat oven to 450°. Bake scones for 13-15 minutes or until golden brown. Allow to cool.
Combine powdered sugar and grapefruit juice to make an icing to your desired consistency. Spread evenly over cooled scones. Top with a slice of kumquat, if desired.