I had conversation with Bedford Building Control when we had a new kitchen fitted. I did the electrics as the fitting company prices were far too high and the spec was minimum. When I told building control that I was doing the electrics and what did they want as a certification from me they got all uppity. I then said that in 1981 my wife and I had built the house ourselves and that I had installed all the services the tone changed and it came down to if I was prepared to sign their standard document of compliance then they would be happy and it need not require paying them any fees as they would not be doing any inspection or testing.
So if I signed a "legal" document that I was happy that the work complied to some standard and was carried out by a person i as house owner considered was competent to do the work then they would accept it.
That was two years ago and they may have changed policy now.