I enjoyed a fantastic Indian lunch on Christmas day at Veeraswamy a few years ago... they are open for Christmas lunch but not dinner. Christmas crackers and other little treats made the lunch extra special. It's just off Regent Street not far north of Piccadilly Circus, so it's definitely walkable. After a nice big lunch we really enjoyed a very quiet walk through St. James's Park. Christmas in London is fantastic, but bring walking shoes!