Finally, after days of frustration I found out how to add a new Apple Watch to my O2 airtime plan, see below. With my watch paired to my phone, I was opening the Watch app on my phone, selecting Mobile Data and pressing the 'Set up mobile data plan' button (I don't remember the exact title) and then nothing much happens, and finally I get an error message saying it could not do this and to look at an o2 link, which is no help. Finally I discovered you need to do the following:
On the iPhone, go to Settings and select Privacy > Motion & Fitness and switch it on.
Then go back to the watch app/Mobile Data/set up mobile data plan and be ready to put your o2 username and password in to the resulting o2 screen. Also, it costs £5 monthly to add the airtime for the Apple Watch, so you'll need to authorise this after entering the o2 user name and password. The phone app will then say 'activating' which can take a day or so, but at that point, you can try doing a phone call direct from the watch to test it.
O2: you need to add the above to the instructions for Apple Watch users for the watch Series 5. Nowhere in the Help area is any of this explained and also the £5 charge is not mentioned anywhere.
