Cost of Apple watch cellular plans on O2, Vodafone and EE?

I am currently with Three mobile and have an Apple Watch with cellular capability. Looking to leave to either O2, Vodafone or EE so that I can use my Apple Watch independently of my phone. My understanding is that these are the only 3 networks that offer an Apple watch cellular plan.

Please can someone tell me to understand what the charges are for adding a data plan on these 3 networks, and the data allowances they offer? Finding it difficult to find this information on the company websites.