IMO there are four main elements to the knowledge you'll need:
1. Technical and theoretical;
2. Practical;
3. Regulatory;
4. Product;
5. Business (if you'll be running one).
Er, five, main elements.
You can pick up all of these on courses, but it's like taking your driving test - you only really start to learn when you're out there in the real world.
Which means that there's also a 6th element, which is how to get yourself out of a seemingly impossible situation with nobody to help you and when all the merchants are shut.
This last one, particalarly, is not taught at college, AFAIK.
As some have said, being fit is going to be important; you don't need a body like Brad Pitt's, but you will need stamina, and forearm and wrist strength, and back health, and titanium knees. Personally, I use the best knee pads that money can buy.
I believe you're already a tradesman, so you have an advantage, especially if you're already self-employed, but attaching yourself to another plumber, at least for a while, is the only way you're going to reach your goal with any speed.
In my case, if it's of any interest, I did one month of half-time work with another plumber (for free), then started trading on my own, while continuing to work him part-time (still for free). This was after already having dabbled in plumbing for about 17 years (did own extension, changed cylinder, connected up new solid fuel boiler, changed css, learned the Water Regs back to front (mostly now forgotten)), and I still knew virtually nothing when it came to those crucial moments.
One tip I would offer - whichever path of learning you choose - buy several metres of copper and plastic tubing, some fittings of all types, some benders, a good blowtorch and soldering gear, a good pair of adjustable wrenches, and spend some happy hours on your own bending and soldering copper and making joints and in all permutations.
Another tip - spend a little time working out for yourself the cost of your time (to the customer) spent farting around working out how to solder something or making on a compression fitting in an awkward space, and compare the cost to that of using more expensive plastic and pushfit materials. Armed with this you'll be able to make a commercial decision about whether or not copper is best in all circumstances, and weigh it up against the aesthetics.