I am an aspirant plumber and enjoying the learning process. Prior to Nov just gone I knew zilch about plumbing - really, I did not know what an isolation valve was or that there was a coil inside my cylinder (I wouldn't even have known it was called a cylinder). Luckily I was able to do a course (during Nov/Dec) to get some basics and have been able to buy a van and tools and stock for that van (I am skint now).
The course was good but I agree with most people that its only a stepping stone and makes one, shall we say, either an "overconfident accident waiting to happen" or, for a more sensible person who knows their limits - "a competent plumber able to look after straight-forward plumbing tasks". It's what you do after basic training that is important and it's what happens inside your head that counts.
Many things, that at first seem insurmountable, can be made to happen if a person has the intelligence and get up and go...
I have an advantage over your average 18 year old (I am in my 40's) - I have somewhere to live and my wife is working extra to feed me and pay the bills whilst I have my little mid life crisis. I also was able to obtain a van and some tools and van stock. Your advantage is that you are young and don't have a wife
Seriously though, you have time on your side... plumbing is not going to go away.
I would suggest that you get a job - any job. Maybe that job involves working in a shop/selling burgers/whatever. Ideally you want a low stress job that doesn't sap ALL your energy and pays your basic rent and food etc. DO NOT build up any overheads (no loans/credit cards/weekly this and that). Treat every penny as if it is extremely valuable and do not buy drink, smokes, anything useless or unnecessary. Buy a few good plumbing tools on Ebay. Only spend money on that which will take you closer to your goal (does that CD take you closer to your goal, do you really need a £20 t-shirt etc.).
In your spare time you research everything you can about plumbing:
You read books
You search the web and research everything to do with plumbing and central heating
You go to builder's merchants/DIY stores and check out every single item they sell to do with plumbing (spend all day in there if you want). If you don't understand an item go back home and look it up on the web.
And you enjoy almost every minute of it... otherwise you will not make it happen and what exactly is the point? What is the point of spending hours a day doing something you hate?
Yesterday I went to EcoBuild at Earl's Court. Train fare was £60 for me but free to get in. I learned a lot...
KNOWLEDGE IS POWER
You ask every person you know if you can investigate their plumbing system - document their systems... what type are they, how are they put together... what's that valve do, why is it there. Crawl around their lofts etc.
You practice your art at home - rank beginners can buy some copper pipe (or hassle a few plumbers for their off cuts etc.) and practice bending soldering etc. The majority of what I did on my course can be done at home. Okay it's all more difficult doing it in your kitchen but it can be done.
Think about it this way... if somebody said that in two years you must be earning £250 a week plumbing or else you will shot dead... then you would make it happen. You can even change that to a higher figure - you can make it happen. You never give up. Every problem can be solved.
You start doing small jobs on your own house, parent's house, mates house - let them buy the bits and you do the work - do it reasonably cheaply and do it bloody well. Turn away anything that is too much but don't be scared to push yourself a bit. Remember - knowledge is power - once you understand systems and concepts then you know how to switch off the hot and cold water and once that bit is out of the way then changing a mixer tap is as easy as buying it and reading the instructions and fitting it. I did this very job yesterday - it actually took me 2 1/2 hours because underneath the sink was like spaghetti junction. I fitted iso valves at same time. Customer supplied tap. £62.50 into bank.
If the will is there then you WILL make it happen. Necessity is the mother of invention.
My father owned an old factory with a septic tank and a pumping station. The pipes from the tank to the main sewage system was his responsibility and leaked into a farmer's field. He got a quote to sort this out and it was something like £5k (multiple leaks and some under a hedge). My father found a guy with a JCB and over a weekend three of us dug down in four places and found four leaks (every time we mended one another seemed to spring up/get worse as pressure increased). We mended the pipes by drying them out with fires then fibreglassing the pipes up (factory used fibreglass). He saved a fortune (I got a few pints out of it lol). Without going into reasons of why and how he also find out something that the local water undertaker had done wrong up at the mains and he sent them a bill for a few thousand for our costs incurred on our pipe - and they sent him a cheque. So he actually made a few grand from the whole thing! I'm rambling now, apologies... but one more interesting thing: the guy in the JCB enjoyed the whole thing so much he set up his own drain clearing company - actually built his own camera on wheels "cos they cost too much" and went on to do very nicely mending underground pipes with a technology where you pump resin down some kind of pipe lining system... last I heard he owns various ventures and lives in a bloody lovely house. He doesn't say too much or talk about it and never moans about anything - he just "gets on with it".
Good luck anyway...