Level 3 is a bit misleading. To be able to read a book and copy the contains quick enough to answer a series of questions is level 3, the C&G 17th edition exam is nothing more than being able to read and remember about 1/2 of the answers to do it in the required time. The C&G 2391 however for inspection and testing is a far more involved test. The 'A' level is a level 3 exam, but my Digital photography was a doddle, the maths however was very different.
Some exams are simply written, others require some demonstration of your skill.
So question one is what is "Electrotechnical systems" I would consider a course which spans over 4 years should give one a lot of knowledge combined with on the job training it does state "Candidates should also have employment within the Electrical Contracting Industry for the provision of the work-based evidence which must include Domestic, Commercial and Industrial installation."
But fitting a 12 volt alarm system would be "Electrotechnical systems" so would fitting Evohome central heating. Today we have nest and hive automating the home, and in the commercial and industrial with have SCADA and many more systems to control machines which would clearly come under "Electrotechnical systems". PLC's and there coms is quite a complex subject and being able to install a PLC system will likely mean high wages.
However one does need to be wary of being a jack of all trades and master of non. I looked for a break down of the course, I got the 726 hours in collage and that it also needs on job training yet some are offering it as a 16 week course. If you are an electricians mate, and you have been doing that for some years, and what you want is to become an electrician then maybe you can do it in 16 weeks?
It does warn about the maths, that's where I struggled, it is odd, I found calculus a problem, but imaginary numbers easy, others on the course found the reverse. OK I was doing a level 5 course, but likely you will still need the maths, 'A' level maths does not cover imaginary numbers, you need further maths 'A' level.
One of the problems was my courses were not always in the right order, so one would be required to do mapping of binary before you were taught the maths of how to do it. Truth tables are not that easy. I thought I was good at maths, until I went back to school, then I realised how what is required has completely changed.
I was asked if I knew logs, yes I said, I had used log tables a lot before the calculator came out, however it was not the use of log tables, it was how to handle numbers to odd powers and completely new to me.
OK I passed, I got my degree in electrical and electronic engineering, but it was not easy, even after 40 odd years in the trade. May be back in the 1970's when I went to collage we did do some of the maths? But I did not remember it.