Hi Michiel van der Velden,
Here is your file back, I do not included the textures, as you have those.
Scene file
https://www.amazon.com/clouddrive/share/dS1i2h3Rray2VwnKNN2i2TPZ8p7fcVxlgWuyIjrdAF1
Each UV case can have some specific needs. The main problem to teach this, is normally over come if you take one UV polygon and move it, rotate it and scale it, to see the results of your actions. Use a plane with 3x3 segments and an image. This playful moving, scaling, rotating will set the base for any further steps. In hands on training I used an “image” with several little images, all distorted to a rhomboid, and asked the people to fit each UV polygon to each picture, so they look like a square. This was in the past 14 years the fastest way to move up the learning threshold of this technique.
Having said all of that, to know the commands in the UV-Edit work certainly helps to solve problems faster.
Here is the screen capture of your file, you can download it.
[one minute]
https://www.amazon.com/clouddrive/share/RDXFecdGEwO4i5E3K4ile12vypaMkMllgnR7g35rbbS
All the best