Purchasing an iPhone 6 in the United States - Using it in the United Kingdom
You must buy the GSM model if you plan to use it in UK. There you could only buy this model because all carriers across Europe using GSM technology. If you visit UK Apple Store, you can only buy that model. Check Tech Specs.

If you are on US, you need a sim card from a carrier that use a GSM network like AT&amp;T or T-Mobile. Check this article if you want to read more: CDMA vs. GSM: What's the Difference?.