Cellular Apple Watch

The Cellular + GPS Apple Watch has an eSIM and shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:

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

The GPS only Apple Watch can do Everything the Cellular model can do AS LONG AS your iPhone is nearby, where nearby is generally somewhere in the same moderately sized home. The Cellular model allows you to leave your iPhone at home while you go for a walk, run, bike ride.

The UK will have a similar add-on charge. But I do not know if it is £5 or £10 or other, but it should be something in that neighborhood.

Check the list of above carriers to find if your carrier supports the Apple Watch, then check your Carrier's website, or call your carrier.

If your carrier does not support the Apple Watch, then either get a GPS only model, or switch carriers, and of course check the new carrier's website for what they charge to add an Apple Watch to your iPhone plan.