Combine Worcestershire sauce, honey, vinegar, lemon pepper, mustard seed, mustard powder, celery salt, and garlic in a large resealable plastic bag; seal and mix ingredients.

Place pork roast in the plastic bag, press air out of bag, and seal. Marinate for 2 hours in the refrigerator, turning the roast occasionally to help coat while marinating.

Meanwhile, prepare grill for indirect heat.

Lightly oil grill grate. Place roast on grill and discard marinade. Cover and cook until internal temperature is 145 degrees F (63 degrees C), 1 1/2 to 2 hours.