The short version: This article from MacWorld UK is wrong. A US-purchased unlocked iPhone 6 works just fine in the UK. (If that's all you wanted to know, I figured I'd spare you the details. If you want to know more, read on…)
First off, you should know: Apple sells a different version of the iPhone in the US (and some other parts of North, Central, and South America) than it does elsewhere in the world. Apple has a snazzy chart explaining, but don't pay too much attention to it. It's a little confusing (if not misleading).
I have an unlocked iPhone 6, model A1549 (GSM). Apple's a little cagey about this, since they don't technically advertise it as an "unlocked" phone. Rather, it's advertised as a T-Mobile "contract free" phone. (If you buy the same phone from T-Mobile, it'll be locked to their service. They'll unlock, I'm told, after you use it on their network for 45 days. But if you buy direct from Apple, it's not locked to any carrier.) In the U.S., the "contract free" (unlocked!) iPhone 6 will work great on T-Mobile, AT&T, and any other GSM carrier.
[Alternatively, you can buy the Verizon CDMA version of the iPhone 6. That phone also has ability to work on GSM networks — since Verizon wants its users to be able to use the phone overseas — and it even comes "unlocked" for usage on those networks. But to get that phone, you need to buy the phone from Verizon, which means you'll be stuck with their network (and a two-year contract) in the U.S. Of course, if you're already a Verizon customer, then no need to buy the full-price contract-free T-Mobile version. Overseas, you have an unlocked GSM phone, so everything that follows applies to you too.]
The non-US version of the iPhone 6 is designed to work with the wider range of LTE bands, some of which are in use by non-US carriers. And if you're going to certain parts of Asia, or you want to use some less-common European carriers, this is important. But here's the kicker: The US iPhone 6 (again, model A1549) does is indeed capable of utilizing the LTE bands that are utilized by the main UK carriers. That means — contrary to what the MacWorld UK article suggests, and what Apple is kind of suggesting on its LTE coverage table — an unlocked US iPhone 6 will work fine, LTE included, with the UK sim card you pick up at Heathrow Airport or at a kiosk in any major British city.
I stopped into a shop at Heathrow Terminal 5. I bought a sim card from British cell carrier Three that offered a month of unlimited data, 3000 texts, and a couple hundred minutes of UK talk time for £30. I popped it into my iPhone, and the guys in the store helped me activate it. I walked out of the terminal (UK LTE isn't quite as ubiquitous as in the US, so I needed to get outside get a decent signal), and there it was: a little "LTE" symbol in my iPhone menubar next to the cell signal icon. The iPhone gave me speeds to match.
(I can't guarantee anything. I'm just posting my own experience here because I found conflicting information when I went searching for answers before traveling.)