Grill (broil) the bacon for 3 minutes, turning once, until crisp. Transfer to a plate, cover and keep warm.
Cut the avocado into slices, across the length, squeeze over the lemon juice to prevent browning, cover and set aside.
Toast the slices of bread. Butter them and spread 4 slices with mayonnaise. Layer the bacon, lettuce, tomatoes and avocado on top. Season with salt and pepper and top with the remaining 4 slices of buttered toast.