Heat the oven to 375°F. Oil a 9-by-5-inch metal loaf pan and line it with a parchment sling.
Soak bread in milk for 5 minutes and mash to a paste. Stir in eggs, onion, Worcestershire, salt and pepper.
Fold beef and pork into the bread mixture with a fork and clean hands just until combined. Transfer to the pan and level gently without pressing hard.
Bake for 45 minutes. Stir ketchup, vinegar and mustard together, then spread over the loaf.
Bake for 20 to 30 minutes more until the center reaches 160°F. Rest in the pan for 10 minutes.
Lift carefully with the sling, letting excess pan juices drain back into the pan, and cut into 8 slices.