Beat butter and garlic in a medium bowl with an electric mixer until creamy. Stir in bacon and 1 tblsp thyme leaves.
Use your fingers to loosen the skin around breast of each chicken. Spread ¼ cup butter evenly under skin and over breasts of each chicken. Divide remaining thyme into four bunches. Place one bunch and two lemon quarters inside each chicken cavity. Secure opening with a toothpick. Secure legs together with kitchen string.
Place chickens into two roasting pans. Drizzle evenly with oil and rub over skin. Season with salt.
Cook in a moderate oven (180C) for 50 to 55 minutes, basting chickens with juices and swapping pans halfway through, until golden and cooked through. Remove from oven. Cover loosely with foil. Rest for 15 minutes.
Discard toothpicks and string before serving. Garnish with thyme. Serve with bay leaves and lemon wedges.