Prepare the pork: Mix 2 tablespoons brown sugar, 2 teaspoons each mustard powder and chile powder, 1 tablespoon salt and 1/4 teaspoon pepper in a bowl. Put the pork on a plate, pat dry and rub all over with the spice mixture. Refrigerate, uncovered, at least 4 hours or overnight.
Whisk the remaining 2 teaspoons brown sugar, 1 teaspoon each mustard powder and chile powder, the Worcestershire sauce, vinegar, 1/4 teaspoon salt, and pepper to taste in a bowl; set aside for brushing.
Preheat the oven to 375 degrees F. Rinse the pork and pat dry. If desired, tie kitchen twine around the pork in a few places to help it keep its shape. Heat the olive oil in a large skillet over medium-high heat. Add the pork, fat-side down, and sear until golden, about 5 minutes. Continue to cook, turning, until golden all over, about 5 more minutes. Transfer to a rimmed baking sheet, fat-side up, and roast 30 minutes. Brush with three-quarters of the vinegar mixture; continue roasting until a thermometer inserted into the center registers 145 degrees F, 15 to 20 more minutes. Transfer to a cutting board and brush with the remaining vinegar mixture. Let rest 15 minutes before slicing.
Meanwhile, make the peas: Bring 6 cups water, the granulated sugar and 1/4 teaspoon salt to a boil in a saucepan. Add the peas, bring to a simmer and cook until tender, 8 to 10 minutes for fresh peas or 3 to 4 minutes for frozen. Drain, transfer to a bowl and keep warm. Add the cream to the pan, bring to a boil and cook until reduced by half, about 3 minutes. Whisk in the butter, then add to the peas and toss. Add the parsley and scallions, season with salt and pepper and toss. Serve with the pork.