In the old days (2005) there was a bit of a do when it came to part P.
Yes truthfully it was a rushed through peice of legislation, true electricians who had worked all their lives where now told (by government) that you could not do your job unless registered.
Then came the choice of governing bodies.
The NICEIC had a very strict and rigid system of enrolement, there was no give or take, you either could do it or you could not.
I wanted to be registered with the best, and bit the bullet, in my abilities etc etc.
Some friends of mine who I had worked with and where like me self employed decided to go to NAPPIT or ELESCA, their only reason at that time was they where afraid of a failure with the NICEIC assesment.
In 2005 that may have been true, nowadays I think anyone who can hold a screwdriver can pass an assesment with the NICEIC, and the other providers are no better.
I would recommend an anual 2391 type exam,a full rewire part done, a full rewire first fix and a customer bank of reviews to pass any assesment.
Only jokin
You now have to decide what type and what sector of work you are doing, then look at the scheme providers who will offer the most help and rewards.
I doubt I will ever leave the NICEIC, although they do hate my abrupt forthcomming comments, however they have been good to me and I to them, apart from the name calling.
Its all down to personal choice, they all do the same. (take your money)