There is no legal requirement to do inspection and testing, but the home must be save for workmen, be it police, fire, ambulance, or carpet fitter at some time some one will need to work in your house so health and safety at work act will apply. Also those considered as not able to look after their own safety children etc. But the 18th edition wiring regulations are not law, but can be used in a court of law.
For DIY it is near impossible to comply with every requirement of HSE, building regulations, and wiring regulations, some rules will be broken, so really what is done is a risk assessment, how likely are you to get it work and produce danger?
The regulations state for a ring final the volt drop at centre should not exceed 11.5 volt, that equates to 106 meters of 2.5 mm² cable, we can work out with maths that is the in coming supply has an impedance of 0.35Ω and the centre socket has an impedance of 0.94Ω line - neutral in both cases then we are right on the limit and there will be 106 meters of cable.
However there is very little today which has a problem with volt drop, fridge/freezer if not inverter type, old audio equipment before they started using switch mode power supplies, I am sure there are more, but as far as danger is concerned it does not really cause a danger as such, breaking the ring final could cause an overload danger and cables could over heat, and for the earth loop impedance going over the limit can result in the magnetic part of the MCB/RCBO not being able to work, but in real terms the RCD will trip with a line - earth fault so may be regulations require it, but is does not really cause danger.
On a new build an electrical firm can use semi-skilled labour who are monitored by an inspector, this has been done throughout time, it is of course how an apprentice learns his trade, however there is one big difference to having a third party tester or LABC tester, no one needs power until tester has visited, in the main once a house is occupied we want power restored within the day.
So in real terms we take a chance, the consumer unit upgrade will mean the ring final it tested to ensure it is a ring, but they will not test volt drop, he may test the earth loop impedance at some random points, but unless there are some glaring faults he/she is unlikely to find them.
As to faults like surface wiring without metal clips very unlikely to find that. Most of the plug in testers with loop test the pass mark is 1.5Ω or higher, and a ring final needs I think 1.36Ω it use to be 1.44Ω and I would have to work it out, but the basic problem is with a 16A supply as used in Europe yes they will show if loop impedance too high for the magnetic part of the MCB/RCBO to work, but in the UK with a 32A supply not really going to show a fault. A 25 amp type B MCB needs 1.75Ω and most of the plug in loop testers will show if above that figure, first light comes on with most between 1.5Ω and 1.7Ω so they will rest a 25 amp supply, but to test if under 1.36Ω you need an expensive loop impedance tester with a actual reading, not simple go/no go lights.
I we want to keep to regulations then there is really no point in DIYNOT as all we would be saying is don't do it. £500 for test gear means it is not financially viable to DIY, my loop impedance tester has failed, I have a insulation tester they are cheap enough around £35 and a low ohm meter, but RCD tester and loop impedance tester stopped working, as to if a RCD tester gives a true result anyway not sure, we were trained to test it with nothing switched on, however we have type AC, A, and if car charger likely type B, and the tester I had would test type AC, but it would not show if it still worked when there was DC on the circuit, so it is like the MCB we don't really test, we just hope it does what it says on the can.