Take a look at the files in lang/po/
We’d need to establish a new en_GB.po file if I remember the country code correctly, and someone would need to go through that file and translate Americanisms into Britishisms(?). Take a look through the file to get a feel for how much work it will be, but keep in mind there are various tools for editing it, you don’t have to edit it purely manually.
A typical text chunk looks like:
#: help.cpp:774
msgid "H= Highway - Like roads, but lined with guard rails."
msgstr ""
Which you want to change to something like:
#: help.cpp:774
msgid "H= Highway - Like roads, but lined with guard rails."
msgstr "H= Motorway - Like roads, but lined with guard rails."
If anyone wants to commit to making a canadian, austrailian, or any other localization (l10n), we can do that.