You'll earn b*gger all as a new starter, probably little more than minimum wage, and maybe less as you'll be a 'trainee' and are therefore exempt from the minimum wage scheme for the first 12 months. Out of this you will probably need to purchase all your own tools (budget about £2000-£3000 for this if you're buying power tools as well).
You should do a City and Guilds 6129 course and get an apprenticeship with a firm. As you're over 19, you will have to pay for the C&G course, unless you can persuade a firm to pay for it for you, which is unlikely. The theory course will cost you about £6000+VAT (reference
http://www.trainingbyreactfast.co.uk/6129_plumbing_training_course.htm others available but similar cost) and last about 10 weeks if you do it full time, or two years if you're on day release. On top of this you will need to build up a practical portfolio, and have onsite assessments which will almost certainly cost you more money.
Sooo...let's say you're working 45 hours a week, and work 40 weeks of the year (52, -10 weeks' training, -8 bank holidays which you'll be lucky to get paid for, and a fudge factor of two days off sick, for which you will receive nothing) and you find a company who will pay you min wage of £5.73 an hour. That's 1800 hours, amounting to a gross income of £10314 in your first year. Out of this you will lose 20% in tax, so let's call that £8000 takehome, less national insurance which I really can't be ar$ed to work out. You've paid £8000-£9000 for tools and tuition, so at best you leave with nothing, and you may make a £1000-£2000 loss in your first year, or £26-£27k less than you're currently earning.
Good luck, and I hope you have a lot of savings and a very understanding bank manager...
EDIT: Incidentally this is just for the first two years, to get you to NVQ2. If you want to go on to NVQ3 and do your gas, unvented HW cylinders, water regs, energy efficiency etc etc then I know for a fact from a friend of mine who's a college lecturer that your 12-18 months doing NVQ3 could cost you somewhere in the order of £6-7k+VAT (his college charge £6800, probably +VAT although not sure on this)