I noticed a couple of years ago that the Aberdeen Angus Steak House restaurants seemed to be open on Christmas Day (not that I'd recommend anyone eat there, ever). I also wonder whether you might have some success in Chinatown. From a quick glance down the OpenTable restaurants with availability at lunch on the 25th, the OP might want to consider The Only Running Footman in Mayfair - although I've not eaten there myself at the 3 course Christmas menu at £65 a head seems ambitiously priced... OpenTable is taking reservations at Joe Allen although the restaurant's own website booking engine suggests it might be closed Christmas Day and Boxing Day.