Why are only certain professions able to countersign passport photos in the UK?

Question

Current law says that passport photographs may only be countersigned by persons within certain professions, and only once such a person has known the applicant for at least two years, specifically the professions listed here:

UK Government information on passport countersignatories

Does anyone know the circumstances surrounding the implementation of this law; particularly with regard to what the historical reasoning was and whether that reasoning is now invalid?

(My personal feeling is this is remarkably out-of-date if not outright offensive and overtly classist, casting unwarranted aspersions on the trust and validity we should place in those who occupy alternative positions, many of which require as much responsibility and are of at least equal value to society. Functionally it restricts those who do not move in middle or upper-middle class circles from travelling freely, or forces them to commit fraud to do so.)

Answer 1

A clue to the reasoning can be found by looking to the example of Canada, which originally had a guarantor system quite similar to the UK's, but changed it greatly in 2007. From the "New Guarantor Policy" section:

Historically, two criteria have been used to assess the appropriateness of the list of professional or occupational groups eligible to act as guarantors. The first criterion is to ensure that all applicants have reasonable access to a guarantor, and the second is to ensure that the guarantor’s eligibility can be readily verified by Passport Canada. For verification purposes, Passport Canada attempted to obtain membership directories of those professional or occupational groups eligible to serve as guarantors in order to validate their eligibility as guarantors.

So the original reasoning was that it's easier to verify a person is who they say they are when they belong to some licensed profession that keeps careful track of its members. If you make up a fictitious dentist as your guarantor, it's more likely to be discovered than if you make up a fictitious retail clerk.

However, this policy ran into many of the problems you note in your question:

The requirement that guarantors be professionals continually generated complaints from the public who perceived the policy to be elitist, and from professionals who were not included in the eligible list. The evolution of professions has created many ambiguous areas, making eligibility difficult to determine, both for the public and Passport Canada. Many groups did not provide Passport Canada with membership directories, requiring Passport Canada staff to contact the association or licensing body to verify the guarantor’s eligibility.

For applications from within Canada, Passport Canada now only requires one's guarantor to be a passport holder; there is no requirement that they hold any particular profession. Additionally, family members were not eligible as guarantors under the old policy, but became eligible under the new policy. Passport Canada now uses its own records to verify the identity of the guarantor.

As it happens, a small vestige of the old guarantor-based system still remains. Passport Canada still allows applicants who are applying from outside Canada to use an occupation-based guarantor for their application. Presumably this is because Canadian passport holders are rarer outside of Canada, and a Canadian citizen who has lived in a foreign country for many years might not know any Canadian passport holders.

Answer 2

As to 'the circumstances surrounding the implementation of this law', according to this 2013 statement by the then Home Secretary Theresa May, there's no law involved:

There is no entitlement to a passport and no statutory right to have access to a passport. The decision to issue, withdraw, or refuse a British passport is at the discretion of the Secretary of State for the Home Department (the Home Secretary) under the Royal Prerogative... Passports are issued when the Home Secretary is satisfied as to: the identity of an applicant; and [a couple of other things not directly relevant to this question].

The list of occupations is not prescribed by law (as a pretty thorough legislation.gov.uk search confirms), and contains whatever the holder for the time being of the office of Home Secretary has indicated will render them "satisfied as to the identity of the applicant". The archive.org Wayback Machine indicates that the list has not changed since January 2016; on the other hand, some remarks in this Commons debate suggest that, in the mid-1980s, GPs were on the list and trade union officials were not.

As to 'particularly with regard to what the historical reasoning was and whether that reasoning is now invalid': as far as I've been able to discover, no Home Secretary has ever publicly stated their reasoning for including or excluding particular occupations (nor indeed their reasoning for having a list of occupations or requiring countersignatures at all); certainly, having searched the historic Hansard website reasonably thoroughly, I feel confident in saying that, if any Home Secretary ever did make a public statement on the matter, they didn't make that statement in Parliament.