Haven't seen the Mix this 'cheap' though probably cheaper if paying in $ using a suitable card/app(I use Halifax Clarity). Don't forget cashback.
The Mix is as close to bezel-less as you're likely to find and comes with a price tag to match its exclusivity.
No band 20 here, O2 users look elsewhere. The other networks will give 4G data though Sam will be here shortly to tell you otherwise. ;)
I've been using the cheaper Mi Max for 6 months and can confirm 4G works as expected on 3.