Add the British English language pack to phpBB.com language section

Based on this support topic: https://www.phpbb.com/community/viewtopic.php?f=466&p=14407901

Many international support sites offer a slightly changed version of phpBB where British English has been replaced with the localized language.

I think it would be good to add the 3.1 version of the British English language pack to the Language page (https://www.phpbb.com/languages/). That way people from those localised IST packages can easily add British English to their board if they want to.