I would be looking for, for works under BS7671 for an electrician with a "Gold" JIB card, that is the nearest thing you will get to a 3rd party validation of the qualifications and experience of an electrician who is a modern entrant to the industry.
That is after the indentured apprentice went, and the NVQ system came in.
As part of that they would have NVQ3, AM2, their BS7671 exam, hopefully 2391-01 or 2394/5 or equivalent if they have recently taken the exam./
2391 is an lifetime qualification by the way.
Things like NICEIC registration are valid for contractors, but not for employees "on the books", as the "enterprise" requires the registration, if the electrician is a self-employed sole trader, then they are the enterprise, if they are on the books, then the employing organisation is the "enterprise" and would need the NICEIC registration.
So, unless you are looking for an electrician to give up their self-employment or their own company and become employed by yourselves, then registration with one of the schemes would not be relevant.
This should cover you for installations and maintenance works on installations under BS7671.
As far as domestic, commercial and insdustrial competencies go, the qualifications cover all of those, and the JIB card would verify these qualifications, it is then down to their experience of these kinds of installations, so that might be an interview thing, and you would need someone competent to quiz them technically on that. Also, their previous employers references, or if you can get recommendations perhaps. A trade test these days would IMHO be a valid thing to do.
Now that covers the BS7671 side.
The next thing you mention is plant.
Do you mean construction plant, e.g. backhoe loaders (aka "JCB", other brands are available), or do you mean plant under the Machinery Directive, e.g. lathes, milling machines, PLC/CNC equipment etc.?
That makes a big difference, please say, and I can guide you more.
There are many excellent electricians who know BS7671 works really well, but know nothing of BS EN 60204-1 for machinery.