Only a court could assess whether someone was competent to carry out certain work should it come to that. So there's not much point in me stating an opinion (as that is all that it would be). Certainly it could include qualifications, experience, training etc.Forgive my ignorance but what makes someone 'competent'?
This section of your post above sounds like any DIYer could cert their work, if no qualifications are needed.
The law (and indeed BS7671), however, requires competence and not any particular qualification(s).