If you're doing Domestic, you want to be with a scheme whether you 'need' to or not. Pain in the rear but makes everything cheaper and easier in the long run.
The schemes (or at least Elecsa, who I'm with) will say you can go with them for PIRs, and you can if you want to use their certificates, but I think that is the only advantage and you'd have to pay for them! Really all you need to do is find someone who will insure you for Professional Indemnity.....
But you say you've been working for someone and are JIB - does that mean you've been mainly doing new 'site work' rather than house bashing, inspection and testing etc. To do PIRs you need a bit of experience in this field to be honest, if you're new to this side of things I'd give it a miss for a year or two whether you can get the insurance or not.
Part P doesn't apply to Commercial or Industrial. There is no compulsory notification scheme for these areas. Make of that what you will.