This wiki page should be instructions on how to set Drupal up to use British English translations. This is marginally more complicated than most, as Drupal by default doesn't know the en-gb language code and has to be told about it.
Some basic bits and pieces can be culled from my blog post here: http://www.drupaler.co.uk/blog/translation-proving-complicated/455
The rest could do with someone more clued up on this than me!
