As far as general employment law is concerned, a boss can just appoint his/her totally unqualified lover to the post if he/she wants to do so, without advertising it at all. (Nepotism in employment isn't illegal)
However the directors of a limited company have a statutory duty to run that company in the best interests of its shareholders and a registered charity must be run with 'good governance' and 'due diligence'. Those considerations imply that there is a duty upon those running the charity to ensure that the post is filled by a person who is well-suited to the job.
However if there's someone already within the organisation who's clearly right for the job they can simply appoint him/her to the post, without advertising it at all. If there are several people in the organisation who could excel in the post (meaning that there would be no need to advertise the post externally) they can simply advertise the post 'in house'. There would only be a requirement to advertise the post externally if it was clear that nobody already in the organisation was right for the job.