All models of Apple Watch will pair with a compatible iPhone and will make all their connections through the paired phone on any network when the phone has data service available (or through WiFi when it is available). Even the cellular version connects through the iPhone if the phone is nearby, standalone cellular is only used when the phone is unreachable.
The GPS+LTE model adds cellular connectivity but it must be setup by your network as a second device on the iPhone calling plan. At this time the only U.K. network offering cellular for Apple Watch is EE and there is no indication the other networks will roll out support anytime soon, so your watch would not be able to have cellular service from 3. The watch would behave exactly like the non-cellular model so it is pointless buying cellular unless you intend to migrate to EE during the life of your new watch.