I would suggest hotel bars and restaurants — as many will be filled with tourists and/or visiting families, most will be open and many serve a proper Christmas brunch and/or dinner. One of my favorite hotel restaurants is the Brasserie Max at the Covent Garden Hotel, but it is probably a few price points above Weatherspoons! Also the Chinatown suggestion is a good one. Christmas is a busy tourist time, so the closer to touristy areas you go, such as Leicester Square and Picadilly, you are more likely to find things that are open.