Hi, I'm pretty much in exactly the same boat as you, I took delivery of my nice shiny new Ceramic 128gb Mi6 last Saturday and I too am in the UK...
I've not actually owned a Xiaomi phone before but I've set many up for friends over the last few years and I've seen them slowly add more and more regions into the official Global ROM but yes, you're right still no United Kingdom option. I've personally chosen France, I used to pick India because they hadn't added many of the European countries... I remember also when United States wasn't there as an option and I was convinced that when they added that one day that they would then too add United Kingdom, but I was shocked a few days ago then to find that US was there but still no UK...
Odd really, seems like it's the only one that's missing now in that big list of Regions...
Doesn't seem to make any difference though, as far as I know it just determines which bloatware to pre install if any, select China for example, I think you get some rubbish apps installed then..?
How you finding it anyway? What phone have you come from?
I've just come from my beloved OnePlus 3 which I've had since launch, fancied getting into this dual camera stuff and managed to get a super duper deal on ******* early morning on Black Friday... To be honest, I'm very very impressed with the Mi6 so far, I'm specifically finding the battery life to be quite amazing! Missing some bits of Oreo though but ahh well...
For the record also, I'm on Virgin network (who use EE) and where I live in getting 4G just fine so I'm not missing Band 20 either...
My next big task is deciding on whether to flash this Xiaomi.EU rom or should I stick with the Official Global? Decisions, Decisions...
Cheers!