In a mixing bowl, combine the flour and sugar. Cut in the cold butter until the mixture resembles coarse crumbs. Gradually add cold water until the dough comes together. Divide into two discs, wrap in plastic, and chill for at least 30 minutes.
Set your oven to 425°F (220°C).
In a large bowl, combine the sliced apples, cinnamon, and toffee sauce. Toss until the apples are evenly coated.
On a floured surface, roll out one disc of dough to fit your pie dish. Gently press into the dish, allowing the excess to hang over the edges.
Pour your apple mixture into the pie crust, mounding it slightly in the center.
Roll out the second disc of dough and place it over the filling. Crimp the edges to seal, and make slits in the top for venting. Brush with the egg wash.
Place the pie on a baking sheet and bake for 15 minutes in the preheated oven. Then, reduce the temperature to 350°F (175°C) and bake for an additional 30-40 minutes.
Allow the pie to cool for at least one hour before slicing.