If you can prove that the work undertaken by another “plumber” was done under your adequate supervision, then as the professional and accountable plumber you are allowed to. We do however like to draw your attention to the fact that once you have signed a PIRB CoC you are not only self-certifying that the work is done correctly and compliant, but YOU are personally and legally accountable for the said works. Signing a CoC without doing the actual work and or under your adequate supervisors is fraught with dangers, so beware.