How about the People Care Association? Johnny Wallington is one of the organisers, they organise christmas lunch, transport to and from for elderly lonely people who would probably not see a soul on Christmas Day.
It is definitely right. I am doing something there on Christmas morning - saw the notice in the local Library. The guy to contact is John on 0207 703 2163. They will be doing Christmas lunch for 400 people. You can volunteer for as much or as little as you want. The lunch is being held at Dulwich Boys College, Dulwich Common. Lunch starts at 13:00 but they will be there from 9.30. You can just turn up and get stuck in with anything needed if you can't get hold of John before tomorrow - you will always be welcome.