Maybe post a flyer at some universities? For example the London School of Economics and many other schools have French clubs that might be interested.
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.
There is a cool ice skating rink at the Somerset House — and also an art museum there at the Courtauld Art Institute that is a best-kept-secret kind of place. The closest tube is Temple, I believe. It is also a nice walk from there to the Strand and up into Covent Garden.
I would DEFINITELY pick up a Time Out (or go to the TimeOut.com website) and look for listings of choral performances. St. Martin-in-the-Fields (Trafalgar Square, take Charing Cross Tube) has some great musical performances, especially around the holidays, and there is a neat cafe called Cafe in the Crypt that is located under the church. One great bit of Christmas music that is performed quite frequently during this season is Britten’s Ceremony of Carols. Messiah will be at St. John’s Smith Square on December 23. You might have to call ahead for a ticket for choir concerts.
For a great, inexpensive hot meal, you should try the noodle bar Wagamama’s. There are several locations. It’s very family friendly.
If you do go to Stonehenge on Christmas, be sure that you get a good guide who can take you around to see some other nearby sights as well, such as the Salisbury Cathedral, and that can give you some good background and history on the drive from London!
There is an overnight train (trenhotel) that runs from Paris Austerlitz station to Barcelona. It departs at 8:30 in the evening and arrives at 8:30 in the morning. The train is called the Trenhotel Joan Miró. You can find out about fares on www.elipsos.com.
I agree — either the Gaudi park or the apartment buildings (name of the one is escaping me at the moment, but they are great).