In a large bowl, mash the chickpeas with a potato masher until mostly smooth. Add kelp, celery, onion, and garlic and stir to combine. Stir in the mayonnaise, mustard, lemon juice, and Worcestershire sauce. Add salt and pepper to taste.
Whisk together the vinaigrette ingredients in a small bowl.
To assemble the sandwich, slice the focaccia in half horizontally. Drizzle the vinaigrette on the cut side of each half. Arrange tomato slices on the bottom half and sprinkle with a little salt. Scatter the olives over the tomato layer. Spread a layer of chickpea salad on the top half and finish with the lettuce and basil.
Cover with the top of the focaccia and press down. If eating immediately, place something heavy (like a pot) on top of the sandwich for about five minutes before cutting. If packing or eating later, wrap the entire focaccia tightly with plastic wrap. When ready to serve, unwrap the sandwich and cut crosswise into individual portions