The majority of PIR's are requested by landlords, estate agents. mortgage providers on a survey advice and insurance companies, espicially in commercial/industrial. The majority of these bodies prefer the tester to be in some sort of competant persons scheme, and 9 times out of 10 will insist on NICEIC, as they have heard of them and not the others.
It is a rarity that a "Mrs Jones" will call and ask you to perform a PIR on her installation. 1) Because she as never heard of it, 2) unlike gas that she knows needs to be tested, why do you need to test the electricity. The majority of PIR's carried out in domestic situations is for a board change, or you may sell one if your working in a house and convince the client that they need one and to spend their money.
I'm not saying that you won't get PIR work, just that as a registered tester with a scheme you might find you can have more opotunities. If NAPIT have not changed their pack I believe that as long as you have the 2391-10 qualification they include the PIR package in their self certification scheme. Perhaps a NAPIT guy can confirm this.