For the sandwich mixture, combine shrimp, crab , mayonnaise, sour cream, dill, pepper and lemon juice together.
Divide mixture evenly between the 12 toasted English muffin halves and place on a 10x15 baking sheet.
Prepare the cheese sauce.
Melt butter in saucepan and stir in flour and seasonings.
Whisk in milk, stirring to blend well and cooking over medium heat until warmed.
Add Old English cheese; whisk mixture until it thickens.
Top each seafood-topped English muffin with a slice of Montery Jack cheese and tomato. Pour cheese sauce over tops of sandwiches and bake at 375 degrees for 10 to 14 minutes, or until cheese is bubbly and seafood is hot.