BookTrust the national organisation that works to promote reading for children and support children's books, is running a scheme for Christmas, donate ten pounds and it will be used to buy a book as a Christmas gift for a child in care or otherwise in need.
www.booktrust.org.uk/support-us/donate-ten-pounds-and-make-a-child-feel-special-this-christmas/
Also if you live need a branch of Blackwell's many run a 'giving tree' at Christmas, you go into a branch and pay £10 and it is used to buy a book for children, the Oxford branch this year is buying books for children/families in hospice care.
I think both are really great schemes. BookTrust runs a letterbox scheme throughout the year posting out books to children in care who may never have received a parcel before ever.