In a small saucepan over medium heat, cook cauliflower in water to cover until just tender, 5 to 10 minutes. Drain and set aside.
In a large pot over medium heat, melt margarine. Stir in onion, garlic and Worcestershire and cook until onion is translucent. Pour in beer and bring to a boil. Pour in chicken broth and return to a boil. Then reduce heat, stir in cauliflower, and heat through.
In a small bowl, combine cornstarch and 3 tablespoons water, stirring to dissolve. Set aside. Stir half-and-half and Cheddar into the soup until the cheese melts. Stir in cornstarch mixture and continue to cook and stir until soup thickens. Serve at once.