Remove roots, outer leaves, and tops from leeks; leave 6 inches of each leek. Cut each in half crosswise. Steam, covered, 8 minutes or until tender. Wrap each leek half in 1 piece of cheese and 1 piece of ham. Place wrapped leeks, seam sides down, in an 8-inch square baking dish.
Preheat oven to 450°.
Place flour in a small saucepan; gradually add milk, stirring with a whisk until blended. Stir in broth. Bring to a boil over medium heat; cook 6 minutes or until thick, stirring constantly with a whisk. Reduce heat; stir in sherry, salt, and pepper. Pour sauce over wrapped leeks, and sprinkle with breadcrumbs. Bake at 450° for 10 minutes or until golden. Arrange 2 wrapped leeks on each English muffin half; spoon sauce evenly over leeks.