The problem is as many will seem to tell you. There seems to be little work out there. If Plumbers are required that badly you would be surprised how quick the controls come off.
In 1963 the year of the big freeze, plumbers on the dole where virtually conscripted into council repair work.
Outside of that, it doesn't matter how well or badly your qualified, its more "Can you do the job!" After all its the product you make that the customer pays for not your qualifications.
You could be a Professor Plumbing Mechanical Engineering but you would probably only get paid the same for washering a tap as a first year Plumber.
Trying to get work, is about selling your wares, like any salesman, not only getting exam passes.
If you notice those who seem to be doing well such as seem to have gone for niche markets. Some specialize in areas others don't do.
The sexy stuff, like central heating repair and installation, seems to be chocker block, with competition, as are kitchen fitting, bathrooms, shower installations and gutter work.
But what about drains?
Commercial and Industrial work?
Sheet work i.e. copper, steel, Nuralite and lead?
Green energies? Voltaic, solar, heat pumps?
Plumbing is a big field, look around for opportunity.
Don't go knocking on doors, go to the factories if you can find any that is and the office blocks looking for work. Its amazing what is out there. Get in with building caretakers and the like and offer bespoke services.