Part P was conceived on two completely false premises - firstly that faulty fixed electrical installations were a major cause of injury - in fact the figures used included portable equipment, which represented teh vast majority of cases.
Secondly, some idiot though that a domestic installation could be tested by just plugging something into the wall.
A major intention was actually to stamp out tax-evading cowboys and "protect" the interests of the industry, who were all too happy to use scare tactics to get their way.
A major problem is that though the requirement is that electrical work be done by "competent" people, the word "competent" has been redefined as "Belonging to an approved trade body".
I can legally do work on my own gas installation, but not many types of electrical stuff, at least not without doing the same Buidling regs paperwork as for major building works.
Of course most people treat this garbage with the respect it deserves and completely ignore it. Even the big DIY stores have long since stopped putting notices about it in the electrical parts sections.