Funnily enough anybody can wire a house out. But it has to be inspected in areas such as the bathroom and kitchen. A council inspector will charge about £100 plus for inspection and certification of your own work. Part P and later revisions tell you that.
The reasoning is, you must follow the appropiate installation regulations to do so.
The unsaid thing is, "How do you know what the regulations are if you have never studied them?". Proof of your knowing the regs, would be an installation that followed the regs. It may be okay chancing things in your own home, except if you have an accident or fire and it turns out to be something you did wrong. But you can't chance it for other people who pay you in good faith beliving you to be an expert.