If you would like to send greeting cards to prisoners this Christmas, here are some options 
Action by Christians Against Torture: Email them for lists at: uk.acat@gmail.com Prisoners name and addresses do not appear on the website
Christian Solidarity Worldwide  A list of names and addresses of persecuted Christians to send greetings cards to is available here 
War Resisters International: Their database of prisoners for peace and conscientious objectors  is part of the Right to Refuse to Kill programme.