I've seen good and bad reports by cps and non cps sparks. You need to be confident you are competent to carry them out, imo that would still mean you carry 2391 or equivalent.
Being non cps does mean you won't always be able to actually correct the non compliances you identify yourself though, domestically of course, if board changes etc are required or other things lead to notifiable works then you have the potentially embarrassing conversation of, "this is wrong but im not allowed to fix it". Thus providing only half of the full service really, you'd need to inform your clients of this fact prior to getting the job imo.