A plumber wouldn’t cut into someone’s supply pipe. That is cadents job. A plumber does plumbing not gas. I do not see where this thread is going or it’s point. I’m out .
With the exception of gas work and work on unvented cylinders, there are NO legal requirement for specific qualifications / accreditation in the UK (unless Scotland is different, don't know).
Some manufacturer's specify that their warranties may be invalid if the device is not installed by someone competent to do so, but that is not a criminal law issue.
One does not have to be licensed so there is nothing stopping ANYONE from setting up as a plumber not matter how appalling their competence. However. There are lots of rules, and training required, to comply with Water Regs, working with things like unvented cylinders and generally complying with building regs.
To be perfectly honest, by far and away the best route in this game, to guarantee both competence and sustainability, is to get work experience and on the job training with an organisation who does a good job and has a good reputation.
IMHO, a so called wet plumber has as much knowledge as a good gas engineer overall. However, the good wet plumbers knowledge is spread widely and a good gas person's is more narrow but much higher (more indepth).
Any fool can set up, and many do. What makes the crucial difference is taking a genuine & responsible attitude from day one. Doing that guarantees your long term success IMHO.