CiviCRM is provided as default with the language English (United States). In order to add English (British), one must download and install the l10 language files from the CiviCRM download area. Documentation on downloading and installing the language files can be found within the Localisation documentation - http://wiki.civicrm.org/confluence/display/CRMDOC/CiviCRM+Localisation
Once the language files are installed, you'll want to tweak the following settings in Administer -> Configure -> Global Settings -> Localization:
Correct display of State/Province (xLON, etc.)
By default CiviCRM abbreviates the state and province when displayed. Whilst that might make sense in Ohio (OH) or New York (NY) it doesn't work in for example London (xLON).
To solve this, go to Administer CiviCRM › Global Settings > Address Settings.
And change the "contact.state_province" token to "contact.state_province_name" for the full name.
Alternatively just consider not using this field since state province does not translate particularly well to the UK in any case.