I asked LG, here is the answer:
The 27UK850W and 27UL850W differ as follows:
- 27UK850: HDR10
- 27UL850: HDR400 + HDR10
- Cabinet colour 27UK850 front silver, back white, base silver
- 27UL850: Matt black front, matt black frame, matt white back, brushed matt silver stand.

To be honest, I don't know exactly what the difference is between the UK and the UL, but probably some small thing. But I would take the UK anyway, because the Ul is not available.

I think comparison with a bit cheaper 650 version can be useful. The number of ports and sound are those that are listed in the descriptions but since I have both, I can share a bit more.
650 version has custom colour warms setting which is quite useful as some people prefer "warm" white while others "cold" one. 850 does not have this setting and by default is very warm. It has some different presets but they produce ugly results. So I have found manual "warms" configuration to be very useful.

From the other hand 850 has usb-c and this is important as many (actually most) laptops will not be able to produce 60 Hz with any other port rather than usb-c. And I find 60Hz to produce softer result when I look at the same picture.

According to the comparison on the LG website, the 27UL850-W has DisplayPort version 1.4, the UK version 1.2. digitec says the UL has version 1.2, the same as the UK. I don't know who is right. By the way, LG lists the UK as "no longer available". The UL also consumes 10% less power.