Preheat the oven to 190 degrees C. Grease a 25x40cm Swiss roll pan.
In a large bowl, stir together 2 cups of flour and baking powder. Cut in 125g butter by pinching between your fingers or using a fork or pastry cutter until the mixture is crumbly. Stir in the egg and milk just until the mixture is moistened. Pat into the bottom of the prepared pan. Spread rhubarb over the crust in an even layer, then sprinkle the dry jelly crystals over the rhubarb.
Prepare the topping by stirring together 2 cups white sugar and 1 cup flour. Cut in 125g of butter as you did for the crust. Sprinkle the topping evenly over the rhubarb.
Bake in the preheated oven until the topping is golden brown and the rhubarb is tender enough to easily pierce with a fork, about 45 minutes. Cool and then cut into bars to serve.