It's me again.
So instead of going through each folder one by one to find where PlayBox stores the movie files, I did this:
1. Downloaded a 390MB movie (Edge of Tomorrow) on my iDevice.
2. I then used this tutorial to find and list files which are above 300MB on my system: https://iosgods.com/topic/20265-how-to-find-files-taking-up-a-lot-of-space-on-your-idevice/
Turned out it stores them on the Documents folder of that app. I have the No-Jailbreak version installed so I'm not sure if it's different for the version on Cydia but if it is, use the tutorial above to find it.
iPad:/var/mobile root# find . -size +300M
./Containers/Data/Application/E92632E2-1603-4559-8727-9A70817F3485/Documents/158531_1453457215.mp4
The subtitles (.srt) and everything else is there too!