I'm going to try and be a bit more positive, if you went by the advice of most here no one would ever come in to this trade. It sounds like you're in a fairly good position in that you're not so reliant on the income and have people you can learn from. It's almost certainly going to take you longer to get to the point you want to, it just takes time. From me actually starting to train to being a self employed gas engineer took around 5/6 years in total, but got there in the end. What i'd say is only take on jobs you're comfortable with at first, say no to stuff that you feel is too much. I did a 20 week gas course (after level 2+3 college), plus going out with engineers to build the portfolio and after that you're still not quite ready until you've got a bit more experience, so i'd say maybe offer to help the guys you know at a low rate. Also once you're qualified go on all the boiler manufacturer's free courses. Be wary of how the course phrase the placement with an engineer, it sounds like you're just mirroring them to learn so should be ok, but i knew a course that guaranteed a job at the end but it was just an interview, which no one ever got the job for.
As others have said signing off other people's work isn't legal so don't base your plan around that. One other thing i'd say is a gas course will teach you nothing other than gas, so you're going to need to learn the overall workings of a heating system, even how a boiler really works from elsewhere. Unless you work for a big firm you can't really just work on gas, people will just tell you their heating doesn't work, they won't know if it's the boiler or something else causing it.
