Maybe I would have to change network by paying off the handset.
The latest Apple Watch is not yet compatible with O2. Currently only available from EE @Anonymous
https://www.macworld.co.uk/news/apple/when-will-apple-watch-3-lte-come-o2-3664181/
"The first thing to mention is that this isn't some sort of artificially created exclusive sweetheart deal set up between EE and Apple - there is, according to EE and corroborated by one other provider, a concrete technological reason why that provider is the only one capable of handling the Apple Watch in the UK.
An EE spokesperson told us that "EE is the only UK network that has the capability to 'share' a number across a hard SIM (in the phone) and the Apple e-SIM (in the watch)."
At present this isn't something O2 can offer - so it isn't simply a question of waiting for an exclusivity deal to expire"
@AnonymousWhilst you won't be able to use it standalone you would still be able to make calls from the watch provided it was within range of a paired iPhone, and you wouldn't need the cellular version for that so it would work out cheaper, though if the phones in range it's better just to pick up the phone. I can count on the fingers of one foot how many times I've made a call on the watch 
They've already said it's not in their plans to support Watch3 so there are no other answers to give?
Maybe it's a different question 
