I have some real doubts about whether these "qualified person" schemes are worthwhile:
- those who take safety seriously will do a generally competent DIY job
- those less inclined to safety standards are similarly likely to ignore regulations anyway
I am certain that scenarios can be created which are beyond the understanding of a normally competent DIYer (as I regard myself) - possibly related to poor earths, shared neutrals, inappropriately sized or routed cables etc.
However if regulations are created to limit injury, death and collateral damage (fire etc), they seem a sledgehammer/walnut solution. Simple alternatives would give far greater public benefit - eg: cycle lanes and mandated helmet wearing, fixing uneven pavements, swimming lessons for all kids.
Even a simple advertising and education campaign alerting the public to the electrical risks unrelated to wiring may have more impact - damaged appliance cables, damaged sockets and plug, wire trip hazards, overloaded sockets, etc.
I worked in a large (1200 staff) office. Routinely someone came round to do PAT testing. I asked how many faulty appliances had been detected over the last few years - none! Time and money being spent reducing an almost non-existent risk to almost zero - daft.