A quick post to tell you that we finally added UTC support to Clocks' and the Shell's World Clocks section. And if you're into it, there's also Anywhere on Earth support.
You will need to have git master versions of libgweather (our cities and timezones database), and gnome-clocks. This feature will land in GNOME 3.28.
Many thanks to Giovanni for coming up with an API he was happy with after I attempted a couple of iterations on one. Enjoy!
Update: As expected, a bug crept in. Thanks to Colin Guthrie for spotting the error in the "Anywhere on Earth" timezone. See this section for the fun we have to deal with.