Even though en-UK is exactly the same as en-US content wise, it is courteous and avoids offending folks out there to have en-UK turned on and redirected from that locale instead of seeing en-US. It will also open the door as a first step toward UK localization
With the way our localization works, we need to actually do the translation before we can enable that locale. While it would be easy to do the en-GB translation, Ali and Igor are currently prioritizing the Thai and Russian translations so we should punt on doing this for now.
I think this is an interesting idea, especially for our English-only-speaking team members, who can't easily get involved with localization. If we had some other en-* locales (en-CA, en-GB) it might help people gain a better understanding of localization within Webmaker. If we do want this, it needs to get done via Transifex, where you can request a new lang/locale to get added. See https://www.transifex.com/projects/p/webmaker/. We've already added Korean and Dutch because of this.
Exactly the same? Certainly not. There are exactly enough differences in phrasing and conjugation to make a proper localisation an excellent idea.