Language/Region selection i18n drupal 7
To get a different content and/or interface for the USA and UK, go to admin/config/regional/language/add :

In predefined language, select English, british. This will add a new "english British" language with en-gb prefix, while default english has en prefix, and will import related .po files.

If you have only en-US and en-GB in your project, when the content is the same for both, simply set it as Language neutral  in the content node submission. 

Maybe you already know the great article series Drupal 7's new multilingual systems compilation. It is a valuable source to get a complete overview of Drupal 7 multilingual setup.