If your finances are in good order you will probably be okay. I did a fast track course last year and have managed to earn enough profit to pay for it, my van, tools, stock, insurances, clothing, tax, etc.
BUT
That was last year
AND
I was very lucky to pick up a three month job and a few other good paying jobs. (March to September).
September to Christmas was okay, but mainly fixing taps, loos and small maintenance type jobs. About 1-2 a week paying £35 to £50 each job.
This year, January was a 2 day job arranged before Xmas. Then nothing.
February was three small jobs (one leaking pipe fixed in an hour), a leaking stop tap (15 minutes) and moving a kitchen sink (most of a day).
March was a week's work putting in a shower, enclosure and tiling. And another job for installing an electrical shower (1/2 day's work). And a third job (1 hour) renewing a cold water cistern ball valve.
Work is picking up a little now the weather's better, but it seems no one is interested in a new bathroom or kitchen or anything new. It's maintenance only on the whole.
If you look at my jobs above, it doesn't exactly pay the mortgage. The competition around here is nearly non existent but then so is the population .... more sheep than people, even after foot and mouth.
Most of my work comes from recommendation and when you're brand new (to anything, not just plumbing) it takes a while for your name to be known, then you've got to be good at your work and be good value to be recommended. It's not easy.
Course (cost £5,000+??) (plus VAT naturally!)
Van (plus VAT) (anything from around £1,500)
Tools (I'd suggest at least £300 will be needed for the first few jobs - buy as required, e.g. don't buy a second tool box until the first is too full)
Insurance - van and public liability
Accountant fees
Stock (at least £100 for some basic fittings, spare loo siphon, ball valve, garden tap, etc)
Total £8,000 for sake of argument.
How to pay for it ... look at my jobs above e.g. change ball valve, one hour. Ball valve costs say £5.00 and you charge £10. Fitting £25 including call out. Gross profit £35.00. Net profit £30.00.
Now the deductions:
Less petrol, less small amount of PTFE tape, less wear on a spanner (they sometimes need replacing too), less electricity for your washing machine to clean your clothes if you get mucky, less tax, less national insurance and less a few other things. Final profit around £10. Remember you need to pay for a service on the van, depreciation, advertising budget if you advertise, telephone bill, etc, etc.
'xcuse my maths ....!! So you need 8 x £10 = £80, 80 x £10 = £800, 800 x £10 = £8,000..... You need 800 customers like that to pay for your initial outlay.
That's why I suggested at the beginning, if your finances can see you through the start (especially in a recession) then go for it. On the brighter side, a recession is about the best time to start a business.
But be warned, it could be an expensive mistake if you haven't access to money to see you through the first year or two.
Choice is yours and I've painted a gloomy picture. But defy me and make a go of it!!
Best of luck!!!
Final note, for basic plumbing (e.g. small repairs) you don't need qualifications. If you don't touch electrics you don't need Part P, if you don't touch unvented hot water cylinders you don't need that part, you don't need to be CORGI registered if you don't touch gas (loads of plumbing without gas!), etc, etc.
If I had my time again and it was this year, I'd go on a short course where I learn to solder and bend pipes and a few principles on how water works around the house and then I'd buy a few books on the subject and pick easier jobs to begin with.
Hope this helps.