Ok. I can confirm that the UK BD framerate is 23.976fps. So if you ensure that your project is set to that framerate and you export at that framerate then there shouldn’t be any playback issues.
The source I used in Final Cut Pro was 24fps, and I imported it into a 24fps project. For whatever reason, It lagged. The only speed it wouldn’t lag at was 60fps. Again, it is not adding interpolated frames unless I tell it to. I think what we have is the best we’re gonna get. I might make a barebones Blu-ray later, and Encore will correct it to the proper framerate without any playback problems. This is the way it’s been for all my projects on here.