iwatch

I have an iwatch series 1 and in the last couple of months the battery has been failing on me a lot quicker than usual, I have noticed it needed an update which I am currently doing and it has been on the 'installing' stage for a couple of hours now. I have been looking to purchase a series 5, however I am struggling to understand whether I would be able to use the Cellular part of the watch as I am on Tesco network, which is provided by o2.