Hi there, it's a little bit more complicated than that. I've looked into this as well as I spend a lot of my year flitting back and forth between the UK and LA.
Here in the UK Apple sells the A1586 iPhone 6 and the A1524 iPhone 6 Plus. Those same models are sold in the US for use only with C Spire, Sprint and US Cellular. The AT&T, T-Mo and other ones use different LTE bands than those in the UK.
Here is a complete list of which phone is available in each territory -
https://forums.imore.com/e?link=http...token=PwszqFez
The biggest thing to consider is to make sure you don't buy a CDMA iPhone in the US as that won't work in the UK.