You forgot BSI...
Speaking as an outsider, NAPIT is the scheme I respect the most, as it is the only one which is based on each individual being assessed, (i.e. none of this Qualified Supervisor looking after a bunch of knuckle-draggers), and the only one which requires 2391.
But JP won't know/care about/understand that.
NICEIC is, I guess the best known to JP, but, IMO, the one that has most shamefully prostituted itself with the "instant electrician" schemes.
But JP won't know/care about/understand that. Until word gets round that some of the people registered by NICEIC are like wild west deputies - just cowboys with a badge.
If you want to take a pride in your organisation, I'd say NAPIT.
If you want something with more recognition, in your place I'd give BSI serious consideration. It may not have the recognition regarding electrical installers that NICEIC or ECA do, and it's registration criteria may be no better, but overall I think that more JPs have heard of the BSI than either of those two, and I think it's a brand that people respect, and the kitemark is a very widely known logo.
My 2p-worth, anyway.