I wasn't sure whether it was worth upgrading this from the Blu-ray I have but it was released in the UK this week and I'm glad I bought it. I haven't compared the picture that much but the sound is vastly better - incredibly powerful.
It’s worth revisiting what Robert Harris said about the Blu-ray. He seemed to say that the 4K scan was done at that time.
https://www.hometheaterforum.com/community/threads/a-few-words-about-tm-the-guns-of-navarone-in-blu-ray.308931/