Ian, and others. The easiest way to convert to en-GB would appear to be simply to do step 4 above - edit WPLANG in config.php After this WordPress will prompt an 'upgrade' to 3.8 en-GB - not immediately though, timing depends on Cron settings?? And the update will install all the language files.

Quoting from http://codex.wordpress.org/Installing_WordPress_in_Your_Language:
>>Note that if the .mo and .po files don't exist for a language code called for in wp-config.php then there is no error message, but the code is still used in language_attributes() This is useful for those of us whose language is similar enough to en_US not to require translation, but who don't want en-US as the language tag in the blog, instead wanting some other variant of English<<