Hi
Yes - Apple Watch Series 3 (GPS + Cellular) can be paired, set up and used for all other features whilst skipping the steps to activate cellular. It will then function like a GPS-only model.
Whether you enable the built-in cellular feature or not, an iPhone 6 or newer model of iPhone is still required, running the latest version of iOS.
Follow the instructions here to pair and set it up, skipping cellular set up if it is offered (a compatible service plan is required):
Set up your Apple Watch - Apple Support
If you do wish to use the built-in cellular feature at a later date, then you will require a carrier plan for your iPhone that specifically includes support for Apple Watch Series 3 (GPS + Cellular), including the carrier activating the device on the plan. As you appear to already be aware, EE is currently the only UK carrier that supports the feature.
An activation fee and/or ongoing monthly cost may apply for the feature - contact the carrier for details: