I found the terms and conditions for EE's smart watch data plan - this looks like the terms for the add-on you'll buy if you bought the watch outright from apple. The pdf is available here:
http://ee.co.uk/content/dam/ee-help/Help-PDFs/smart-watch-pay-monthly-simo-plan-terms-2017-v1.pdf
I've skimmed through it and the stand-out feature is that it's a
30-day contract. If I'm reading it right, you can add it and take it off as you please, presumably just like the data plans for the iPad.
I really hope I'm right on this - I won't always need data on the watch so turning it off when I'm out the country (it doesn't support roaming) so this looks like a good deal.