Thanks for your valuable advice.
I understand that this is an industry of life long learning. My main fear was exactly what you said: failing the interview due to wrong answers. You have one chance to pass your interview and your abilities to really show any progress are limited (but you can show a willingness to learn, that's true).
I am definitely doing some reading. I got hold of the Oxford Aviation PPL theory which I am definitely reading a few times before going to the interviews. I don't really know if actually going the extra mile to get the written exams before the interviews would be an advantage of a liability.