In a small saucepan, cover the prunes with the port and bring to a simmer. Remove the pan from the heat and let stand until the prunes soften, about 25 minutes. Pour the prunes and port into a large, shallow dish. Add the garlic, lemon juice and zest and a large pinch each of salt and pepper. Add the pork roast and turn to coat. Let marinate at room temperature for 4 hours, turning the meat often.
Preheat the oven to 400°. Remove the prunes from the marinade. Strain the marinade into a small saucepan and reserve. Roll up the anchovy fillets and stuff each one into a prune. Make a deep lengthwise slit in the pork loin, leaving about 1 inch of meat attached. Open the roast like a book and season with salt and pepper. Arrange the stuffed prunes in a row along the slit; close the roast and tie it at 2-inch intervals with kitchen string. Season the roast all over with salt and pepper.
In a large skillet, heat the olive oil until shimmering. Add the roast, fat side down, and brown it over moderate heat on 4 sides, about 1 minute per side. Set the roast in a roasting pan, fat side up. Roast for about 55 minutes; if the pan juices begin to look very dark, add 2 tablespoons of water to the pan. The roast is done when an instant-read thermometer inserted in the center of the meat registers 145°. Transfer the roast to a carving board, cover loosely with foil and let rest for 10 minutes.
Scrape out any blackened bits in the roasting pan. Set the pan over moderately low heat and stir in the flour to make a smooth paste. Slowly whisk in the reserved marinade and simmer, whisking, until the sauce is smooth and thickened, about 3 minutes. Pour the sauce into a clean saucepan and season with salt and pepper.
Untie the roast and slice it 1/2 inch thick. Pour any juices into the sauce and reheat before serving it with a pork roast.
The anchovies and prunes that enliven the roast pork call for a rich, spicy wine.