Apple Watch with UK network provider

There are 2 Apple Watch cellular radios

• Canada, U.S. Mexico, Puerto Rico

• The rest of the world

Carriers will not attempt to connect to an Apple Watch that does not use the correct radio.

If you do not get your Apple Watch in your own country, you may have to return the Apple Watch to the country where purchased to get warranty service.

If you do not get your Apple Watch in your own country, there may be country specific restrictions in the installed Apple Watch software that prevent features from working when you get home.

https://www.apple.com/watchos/feature-availability

As has been mentioned, there is no roaming, so even if your UK carrier will support the Apple Watch while you are in the UK, the Apple Watch will not use Roaming.

And even if it could use Roaming there is the 2 different radios and cellular networks. A UK Apple Watch will not talk to a U.S. cellular network.

You can use a UK Apple Watch in the U.S. as if it was a GPS ONLY Apple Watch, which will do everything a connected cellular model can do As Long As the iPhone is nearby.

No, if it's Watch GPS + Cellular model.

Roaming is not supported.

The cellular Watch will not work with a U.K Sim in U.S.

For cellular to work, both Watch and iPhone must be from the same U.K. carrier (phone number sharing)

And roaming is not supported.