There are many routes to become an electrician. This is why there is no paper qualification required to become an electrician. However to work on domestic they or at least their company really needs to be a scheme member, without being a member of a scheme they can't self certify the work. With commercial the required qualifications only really show they can work on sites without being injured or killed mainly by industrial processes not by electricity.
But there is nothing stopping a company employing me to do electrical work without ever seeing my qualifications, all they need to do is look on the electro role to see that I am British and am allowed to work in this country, or see my passport. At my age I could have done an apprenticeship without ever going to collage and so could in theroy be a really good electrician without holding anything other than my apprenticeship deeds.
In real terms I would likely have returned to collage to show I can read a book, the C&G2382 shows I could extract information from the regulations book quick enough to pass a test. This means I must remember some of the book as there is not enough time to look up every question. In order to inspect and test except in Scotland I don't need any qualifications, although C&G2391 or C&G2392 is considered as the qualification required to show you know how to inspect and test a fixed installation. In Scotland you have to show resent learning I think, but only in Scotland is that true. For the appliance the C&G2377 is split into two exams one for the testing and one for the management of the paperwork, the problem is it is inspection and testing even if of in service electrical equipment so a simple question of "Have you got your inspection and testing" could correctly be answered yes with either C&G2391 or C&G2377 the former is quite hard to pass, the latter is quite easy, all the City & Guilds exams listed are all considered level 3 even when all it means is you can read a book.
The C&G exams have changed through the years, so those electricians who started years ago will have never taken a NVQ as they simply did not exist. And although the NVQ was intended to be a hands on course of education in real terms I have met many who are good passing tests, but useless doing their job. I have never needed personal insurance, but my son has, and he did not need to show his qualifications. In theroy inspection and testing is a professional option, so should require Professional Indemnity Insurance, but often the Public liberty Insurance has a clause, which covers the electricians inspection and testing.
So as a commercial employer you need to ask previous employers just like with any other trade, with domestic you need to select some one who is a scheme member. As one rises in the profession, you can gain qualifications which reflect the job you are doing. I needed a level 5 qualification and a recommendation to become a member of the IET, I am sure there other ways in? if I was interested as I get positions of responsibility I can request the IET to recognise this, and I would gain letters behind my name to show how well I had done. I have never bothered, mainly as it costs more the higher you raise though the ranks.
Odd as it may seem my degree did not teach me how to do house wiring or any part of the regulations. So an electrical engineer could have no idea on how to wire a house. Fact I did my degree after becoming an electrician yes I do, but I could have specialised in say machine control programming PLC's etc, and have no idea how to wire a house. Yet I could have gained a whole string of letters behind or even in front of my name.
We are still members of the EU so Polish and German electricians can work in this country, it would be expected that they take the C&G2382 to show they know our regulations, but a company could employ them and use one guy with his C&G2391 to test and inspect may jobs being done by even electricians mates or apprentices. Who could have only just started. So as a domestic user, your only way to be sure they have some level of control is to use scheme members. In spite of having 2382, 2391, and 2377 and a degree in electrical engineering I don't have an NVQ of any level, they did not exist when I did my apprenticeship. And since mainly commercial I have never had a JIB card, however I have met electricians mates with a full JIB card, hard to get now, but when JIB started they issued cards like confetti so yes shows some thing with 25 years old, but for a 60 year old it means nothing.