Hello, Logic4Training seems to be very trustworthy. 4.9 stars on Trustpilot and 97% positive feedback.
I used them for a 17th edition course a few years ago and the company I did my last two ACS's with and G3 unvented HWSS use Logic4training's books/materials which I believe is now LCL Awards on the books and certificates.
The issue with all of these these courses though is that, unless you have been shown in a real-life apprentice-like environment by an experiences tradesman how to do many many tasks and overcome many problems, you will have a bit of a nightmare learning it all yourself by trial and error.
There are so many possibilities of what you will come across on the job in real life, you'll have to stick with it for around 7 years to be comfortable and even then you'll still come across new things but at least you would have the experience by then to deal with it.
Sometimes you only need to go to a different town to come across an estate with a whole load of houses done differently to what you've seen before with different heating and hot water systems to any others in your region!
If you are prepared to stick with it, work very hard, be very stressed out and finally get to the light at the end of the tunnel, then it is possible. Some people do manage it.