Combine the flour, sugar, baking powder and salt in the bowl of a food processor and pulse once. Sprinkle the butter evenly on top. Pulse, pulse, pulse - in 1-second bursts - until the mixture looks like grated Parmesan cheese.
Add 1 1/2 cups of the Cheddar and pulse, pulse, pulse, while slowly drizzling in the cream, until the dough starts to come together.
Form the dough into a disk, and transfer to a floured work space. Gently roll the dough with a rolling pin into a 1-inch-thick rectangle. Cut out 35 pieces of dough, using a 1-inch-diameter ring mold, and transfer to a parchment-paper-lined sheet pan.
Bake, turning the pan halfway through, until golden brown and cooked through, 14 to 15 minutes. Let cool.
Meanwhile, combine the malt vinegar and apples in a medium mixing bowl. Stir in the creme fraiche, scallions and remaining Cheddar, and mix well.
When the scones are cool enough to handle, halve them equatorially. Place 1 slice of the corned beef on the bottom half of the scone, and top with the apple, cheese and scallions, followed by the top half of the scone. Repeat until all of the scones are filled.