My only point to add is do make sure that you have your place lined up in advance, rather than walking the streets, as you will find the majority of places closed on Christmas day. Those that are available, including the restaurants in major hotels, have often taken reservations long in advance from those who don't want to do the cooking on the day themselves. Traditionally this involves an over-long "dinner" at lunchtime, probably spanning from 12 noon to around 4 pm, something of a houseparty atmosphere, and the bulk of the diners going for the "traditional" style of food. Think Thanksgiving in the USA. There is normally special and very expensive pricing compared to the normal menu. It is very unusual for places to be open in the evening after all this.

The stretch north from Marble Arch along Edgware Road, which has many Middle Eastern restaurants, is a good idea, they are one of the few places you can expect something to be open. I have to say that Hummus and Pitta Bread is, however, not many British people's idea of dinner on Christmas Day !

If even mildly religious, you might like to go to the Christmas Eve midnight service at Westminster Abbey (walkable from your place) or one of the other major churches. Get there by about 11 pm, they do actually get full up. Finishes about 1 am. Then walk back through the quietest streets you will ever experience in London. Oh, and hope it's snowing !