Melt the butter in a saucepan over medium heat. Whisk in the flour and cook for 2 minutes.
Slowly whisk in the broth and cooking liquid. Add Worcestershire, thyme and pepper and simmer for 5 minutes until lightly thickened.
Pour the gravy into a slow cooker and add the cooked pot roast. Cover and warm on low for 30 to 40 minutes, until the beef reaches 165°F.
Meanwhile, boil the frozen noodles according to their package directions and drain them well.
Divide the noodles among bowls and ladle the hot beef and gravy over them. Add parsley only if using and serve immediately.