When I decided to upgrade my qualifications at that time the local collage ran a series of three courses, the 16th Edition as it was then was how to read a book, simple, then the PAT testing was more hands on and a bit more involved, but still reasonable simple, then the 12 week course giving one the C&G 2391 on how to inspect and test an installation, although everyone on the course was an electrician to start with, there were a couple who were finding it hard, and it was rather involved, we had already done the other two so in all 24 weeks in college at 3 hours a week in night class, I am told everyone did pass, but nationally the pass rate was around 54% clearly not easy, specially since most taking the course were already electricians.
My point it is unreasonable to expect any DIY person to learn the skills required to be able to inspect and test their own house. I remember that exam well, I tested a board and the results were within the limits set, however I said there is a fault I need to look further, this was simply because experience told me such a short run of cable would not give those readings, and I was right, I found resistors soldered into the cables so had the sockets been used then the resistors would have melted the trunking and likely causes a fire. Seems the resistors would put there to simulate a larger installation, however they would have caused a fire and testing by trained personnel should have found them.
So plugging in a loop impedance tester and seeing the reading is 1.02 Ω when the pass mark for the MCB used is 1.37 Ω does not mean it's OK. There is more to it than that, for a start although 1.37 Ω may mean the MCB will trip in the required time, it does not mean the volt drop will be within limits, in fact very likely it is not within limits. I would expect the line - neutral reading to be below 1 Ω for the volt drop to comply, depending of course on the reading on incoming supply.
I watched an electrician move a light, in doing so he used extra cable, so to lights around 2 meters apart had around 8 meters of cable, nothing wrong with that, but it resulted in readings which at first glance did not seem correct, but over the years we get a feel of what is right and what is wrong, nothing to do with regulations, it is just one has tested so many times, you can guess what the reading should be, and if it's not what your guess is telling you it should be, you look for a fault.
Now with a full rewire the DIY guy knows what he has installed so testing is for him not that hard, but where using some existing wiring then testing and inspecting becomes really important, I got my mothers house rewired, I did not have the time to DIY the job, needed doing while she was in a care home for 4 weeks so paid for a firm to do the work. During the work I found an assortment of my fathers work, some times he had actually followed safe zones, other times he had not, there were 7/0.036 cables joined to 2.5 mm² cables and then back to 7/0.036 then to 4 mm² with junction boxes galore. Even some singles in parts. They may have tested OK but inspection showed it needed a rewire. It seems the original ring had 5 sockets 1954 at least that's what my dad told me. He had added so much over the years even before he died he could not have told you what was fed from what, we found FCU's covered by wall cupboards and in one case 3 FCU feeding each other. At around 0.010 Ω detecting a fuse in a circuit is not easy.
I really can't see how a DIY guy would be able even with all the courses to do a part rewire, it needs so much experience which simply can't be gained from books.