Hi
On your iPhone, in the Clock app, tap on the World Clock tab, then tap the + sign (top right), then scroll to UTC and tap it to add it to your list of World Clock locations.
Then, when customising a watch face on your Apple Watch that supports complications (via a firm press on the watch face > tap Customize):
- Swipe left until you can select individual face features > Tap the one you’d like to use for a world clock > Turn the Digital Crown to choose a city (it offers each location that you have set up on your iPhone, which now include UTC time) > When finished, press the Digital Crown.
World Clock is available on these watch faces: Chronograph, Color, Mickey Mouse, Modular, Simple, and Utility (and you can add more than one on some of them).