HBO GO is now live on Xbox One for US residents. This allows HBO subscribers access to every episode of every TV show HBO has ever aired, including such series as Game of Thrones, The Wire, and Enlightened.
It also contains a rotating cycle of movies, including recent hits like The Lego Movie and 12 Years a Slave, along with some not-so-recent not-so-hits like Cop and a Half and Dunston Checks in (as is part of the current selection).
When asked whether the service would be available in the UK, a Microsoft representative suggested it wouldn't with the following statement: "HBO GO on Xbox One is available to all HBO subscribers in the U.S. who authenticate through participating cable providers. We've nothing to add beyond that."