Apple watch

The answer to that is generally no.

Here is a list of cellular carriers that support the Apple Watch. You would have to check with the carrier to see which of their plans include Apple Watch support.

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

Your Apple Watch has to be connected to the same cellular plan as your iPhone. I'm not sure if PAYG accounts can do that. Check with your carrier, since they would have to support activation of cellular on the Apple Watch.