In a large resealable plastic bag, combine the Worcestershire sauce, honey, vinegar, mustard seed, mustard powder, lemon pepper, celery salt, and garlic; seal, and mix ingredients. Place the 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.
Lightly oil grill grate. Place roast on grill, and discard marinade. Cover, and cook for 1 1/2 to 2 hours, or until internal temperature is 145 degrees F (63 degrees C).