So I've copied this from a post from @MI5 (who knows his stuff) and this is what it says:
Set up the data plan from the watch setup routine and it will walk you through buying an airtime plan.
https://www.o2.co.uk/help/phones-sims-and-devices/apple-watch
Open the Watch app on your iPhone and tap on ‘Mobile Data’.
Then tap ‘Set up mobile data’. You’ll be prompted to sign in with your My O2 username and password.
Once you’ve signed in, select your Apple Watch plan. Follow the steps on the screen to set up your Apple Watch Airtime Plan.
It can take up to 24 hours to activate, but sometimes it might take a little longer.
To check if your new Airtime Plan is active, open the Watch app on your iPhone and tap on ‘Mobile Data’. In the ‘Data Plan’ section, if you see O2 with the word ‘activating’ then you’ll have to wait a little longer for set up to complete. If you see O2 with an orange ‘i’ next to it, then you’re all set up.
If you get an error message at any point, 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.
You also need to have iData enabled on your account and pay bills by direct debit.
If you've done all that and it still doesn't work then post back here @markrduk and we can get some more help lined up...
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
