[Xiaomi Mi MIX  Questions & Answers] satishp:
Considering there are only two variants of the device (4/128 & 6/256) available, I don't see how a "UK" buyer's opinion would be any different from that of others.

Now, to answer your questions, most buyers here, have bought from Chinavasion and Flosmall as they have ready stock and promise fast shipping. Wondamobile is also recommended by many and their prices are inclusive of import charges. The price you pay on Wondamobile is the final price. Flosmall also offers a similar option if you choose to opt for their "EU customs insurance". If you opt for this, they claim that they will refund any import charges paid to customs minus DHL's clearance processing charges, within 1 business day of submitting the customs invoice to them.

2. It will work straight out of the box without the need for any flashing as most vendors ship it with a customized ROM with Google services pre-installed. There are no issues with the vendor ROM but it is recommended that you switch to the EU beta ROM or one of the custom ROMs available, if you need updates.

3. Android Pay should work on phones with bootloader locked. If your phone comes with an unlocked bootloader, its fairly easy to relock it using Fastboot.

And when you compare the Mix with the Mate 9,

Mate 9 - Pros

The Mate 9 has a slightly better quality IPS display. (Same FHD resolution, but higher pixel density due to smaller 5.9inch screen, slightly better contrast and colors.) - Source: GSMArena Display Tests. The Perceived quality difference is almost negligible though. The Mix also has one of the best FHD IPS panels in the market.

I have compared both displays side by side and both look equally great. So its up to you to decide whether numbers matter.

The Mate 9 has one of the best cameras on a flagship phone. So if you are someone who takes a lot of photos, the Mix isn't for you.

Expandable Storage via Microsd on the Mate 9, but do you need more than 256 GB of UFS 2.1 Fast onboard storage on the Mix?

Mate 9 - Cons

The Mate 9 has a Kirin 960 processor. While CPU speed is close to the Snapdragon 821, the GPU is slightly weaker. The main drawback however, is that the Kirin Chipset is not as developer friendly as a Snapdragon.

There are fewer custom ROMS for Custom designed chipsets like Huawei's Kirin and Samsung's Exynos. Any stable custom ROMS you find will be based on the base OS.

Forget stable AOSP\CM\Lineage OS development. Most developers prefer Snapdragon.

Once\If Xiaomi releases the kernel sources for the Mix, the device will definitely be open to development. This is one of the reasons why I chose the Mix over the Mate 9. I've used Samsung phones before and none of them had decent custom ROMS due to the Exynos chipset. So I wanted a Snapdragon based device as development makes your device more future proof.

The International Version of the Mate 9, MHA-L29 only has 4GB RAM at the moment. There are rumours of a 6GB L29 Variant though. As of now, only the Chinese variant (MHA- AL00) has 6GB RAM and 128GB Storage. While 4GB of RAM is more than sufficient, almost every flagship this year will come with 6GB as a minimum. So the Mix is more future-proof.

Smaller battery (4000maH) compared to the 4400mah battery of the Mix. And the Mix has excellent battery life.

While the design of the Mate 9 is decent, it isn't a head-turner like the Mix!

Hope this helps.

Cheers!