To cook beef in a slow cooker in the UK, start by selecting a suitable joint of beef like topside, brisket, or silverside. Dry the beef and season it with salt, pepper, and herbs or spices such as thyme and garlic powder. Optionally, brown the beef in a hot pan for extra flavor before placing it in the slow cooker. Add vegetables like onions, carrots, and potatoes around the beef. Pour in beef stock or a combination of stock and red wine to cover some of the meat and vegetables. Set the slow cooker on high for about 5-6 hours or low for 7-9 hours until the meat is tender. Towards the end, remove the beef and vegetables to rest, then thicken the cooking liquid with a slurry made from cornflour mixed with a little water to make a gravy. Serve sliced beef with the gravy and hearty sides such as roast potatoes or mashed potatoes. This method is energy-efficient, convenient, and results in tender, flavorful beef with a rich gravy.