Cook the bacon in a frying pan over medium heat until crisp. Drain the cooked bacon on paper towels and set aside.
Spread one side of each slice of bread with some of the mayonnaise. Place a lettuce leaf on the mayonnaise on four slices of the bread. Top each with some of the sliced tomato and bacon. Add another slice of bread to each, with the mayonnaise side up. Top with the remaining the lettuce leaves. Divide the turkey or chicken among the sandwiches and top with the remaining bread, with the mayonnaise side down. Cut the sandwiches in half diagonally. Thread an olive and a gherkin (if using) onto each of eight wooden picks and poke one pick into each sandwich half.