You have hit the nail on the head there Glenda. It is about competency.
Like having electrical tools and equipment 'PAT Tested,' it can be done by any competent person, not necessarily by a qualified electrician or electrical engineer and there is no certificate of competency required.
If someone like a letting agent requires a certificate, I see nothing wrong in drafting your own for a situation like this and getting the workman to sign it as a competent person carrying out the work.
Obviously, as previously mentioned, this would probably only apply to minor repairs and replacements, not to actual installations or major upgrades.