cellular watch

iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

Sky is not listed as a carrier that support the Apple Watch.

A cellular Apple Watch shares the iPhone's phone number and the iPhone carriers that support the Apple Watch need to be able to provide that service. Supporting carriers then charge a per month fee for sharing the phone number. In the U.S. it is $10 per month.

A carrier may not support the Apple Watch because they do not have the tech for sharing an iPhone phone number, or they do not want to, or there is a contractual limitation with the carrier they use for their service, or there is a contractual issue with Apple.

You will have to ask Sky if they plan on supporting the Apple Watch.

Otherwise either get a GPS only model that will do Everything the cellular model can doas long asyour iPhone is nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction).

Or switch carriers if you must be able to leave your iPhone at home while you go for a walk, run, bike ride, etc...

Ask the folks at Sky. Or O2.

There’s no SIM with the Apple Watch cellular; it’s an eSIM.

Here’s some O2 reading on this topic:

https://www.o2.co.uk/help/phones-sims-and-devices/apple-watch