Cook the bacon in a large skillet over medium heat, turning occasionally, until crisp, about 8 minutes. Drain on paper towels, then break each strip in half.
Meanwhile, combine the mayonnaise, lemon juice, herbs and celery in a large bowl; stir in the chicken. Season with salt and pepper.
Toast the bread and spread with mayonnaise. Divide half of the chicken salad among 4 slices of toast. Top each with some potato chips, 1 piece of bacon, 1 tomato slice, some lettuce, watercress and another slice of toast. Repeat with remaining ingredients to make 4 double-decker sandwiches. Gently press each sandwich together and cut in half. Serve with pickles and more potato chips.