New I watch

You can use an Apple Watch with an iPhone 6. I do not believe, however, that you can get cellular service on the Apple Watch from Gif Gaf. So, save yourself the money and don't buy the cellular version of the Apple Watch.

As long as your iPhone is nearby, you can do EVERYTHING on a GPS only Apple Watch that you can do with a cellular model. Please read the following

https://support.apple.com/en-us/HT205547

Make sure you read all the footnotes.

I have a GPS only Apple Watch and an iPhone 6s. So does my Wife.

Gifgaff is a UK network reseller. They don’t support Apple Watch and as far as I know none of the UK networks support Apple Watch on pay as you go (prepay) accounts.

That’s a good point. I had taken the question as relating to a cellular version of Apple Watch - maybe incorrectly. To clarify, my comment above only refers to network support for the standalone cellular service on Apple Watch when the iPhone is not near.

As BobHarris says, the non-cellular (GPS) watch will work by communicating through the paired iPhone, which means it will work on any network where the phone has a data service.