The list of acceptable professions can be found on pages 25 and 26 of this document
Nationality Policy

The Home Office has said the following in response to enquiries regarding referees:

Note the list is not an exhaustive list; therefore, an individual’s profession may not be listed as acceptable although they may fall under the category of a person of any nationality who is of professional standing or a member of a professional association. If the caseworker is not satisfied with your referee, they may request another.

Furthermore, a referee who is a professional person should have had dealings with the applicant otherwise than in a professional capacity. However, we would expect there to have been regular face-to-face contact between the applicant and his or her referees over the three-year period, such that both referees could confidently assert that the photograph submitted with the form was a true likeness of the person making the application.

Referees should also be prepared to inform us of anything known to them about the applicant that might cast doubt on his or her eligibility for British citizenship, but it is accepted that referees will not necessarily be aware of any disqualifying factor or characteristic, particularly where their dealings with the applicant were solely in a professional capacity.

The requirement to nominate suitably-qualified referees is not a statutory one, and where it presents genuine difficulty for an applicant on account of his or her lifestyle or other circumstances this should be explained in the space provided for additional information on the application form.
Although GPs were removed from the list because they were receiving to many requests of this nature, the list is not exhaustive, and a GP or any other professional not on the list, can still act as a referee if they meet the requirements. This means you can ask a GP to be your referee if you know them in another capacity, but you shouldn't really ask your GP to be a referee.

Retired professionals can be used as referees where the list of acceptable professions specifically mentions it, i.e.: “Officer of the armed services (active or retired).” Also, civil servants who were permanent when working.

If the caseworker thinks one of your referees is not suitable, they will contact you asking to provide another referee.