would I have any issues connecting to UK cellular provider with a US purchased watch (GPS + cellular)
The UK providers will not touch the U.S. Apple Watch. And the U.S. Providers will not touch the UK Apple Watch. They do not want the headache of the only frequency at your house happens to be the one your Apple Watch does not support. And Apple will not help to resolve any issues.
Yes they operate on different frequencies.
That doesn't appear to be true. The USA and UK bands are, for the most part, identical, and common GSM and LTE bands are supported by both. I'd like a definite answer to this.
Apple Watch 6
