Have the 4k disc and did a quick a/b comparison to the Criterion Blu-ray and the image quality on the 4K disc looks great with deep colors and more picture info on all four sides. The image did not look too dark to me but on my tv i do have the gamma setting up a little more to make the picture...
I have a few Arrow Blu-ray releases like The Day of the Jackal, Flash Gordan 4K, The Voice of the Moon (Fellini), The Andromedia Strain, and Cinema Paradiso 4K and happy with all of them. I had no problems buying the UK versions and did buy them at Amazon UK until the price of shipping went up.