I am a part p approved contractor registered through the niceic.
Part p was introduced in January 2005, its part p of the building regulations which range from A to P. Part P covers electrical installations in the home.
You can still carry out your own wiring or use an electrician not registered with a part p scheme but you need building control approval for the majority of work carried out. Building control charge a significant fee for inspecting your wiring and its likely it will not meet their standards. When you come to sell your house your vendors solicitor will ask for part p compliance and a certificate for any electrical work carried out after January 2005.
I can self certify and notify my work to building control.