These pieces of secondary legislation, as their names suggest, are made under the authority of the Town and Country Planning Act 1990. Several sections of the Act empower regulations to be made by "the Secretary of State", e.g. in s.220(1) we read
Regulations under this Act shall make provision for restricting or regulating the display of advertisements so far as appears to the Secretary of State to be expedient in the interests of amenity or public safety.
and the full Parliamentary procedure for making them is spelled out in s.333. This comports with the regulatory preambles, e.g. for the 2007 regulations, saying:
The Secretary of State for Communities and Local Government, in exercise of the powers conferred by sections 220, 221, 223(1), 224(3) and 333(1) of the Town and Country Planning Act 1990, makes the following Regulations
Note that the Act just said "the Secretary of State", whereas we have just read about a specific Secretary.
What's going on here is that primary legislation just wants to say "whoever in Cabinet has a job that is most relevant", since functions get transferred around, departments created or abolished, etc., and nobody really wants to update a zillion Acts every time that happens. Instead, there is a framework pattern where powers will be given to "the Secretary of State" generally, but will be executed by a specific one according to the division of responsibilities in the government of the day. That will sometimes be just a matter of agreement within Cabinet, but at other times be encoded in secondary legislation - a "Transfer of Functions Order". Those Orders are also needed to handle special situations like -
- transferring property and legal obligations when departments are created, merged, split or abolished
- making amendments to legislation which did happen to mention a specific minister
- ministers who are not a "Secretary of State" as such, but hold another ministerial title, such as "Lord Privy Seal"
These Orders are made under the Ministers of the Crown Act 1975, and are in the form of orders of the King on the advice of the Privy Council - which is to say, that the Prime Minister decides who does which jobs. Historically, this is something of an accident, since the number of Secretaries of State has increased along with the scope of government, and it was convenient to appoint lots of people to the same formal office rather than invent fresh jobs - especially in the days before ministers were paid.
In any case, flag decisions would currently be made under the authority of the minister responsible for planning affairs, the Secretary of State for Levelling Up, Housing and Communities, Michael Gove. That comes from a 2021 order when that position was created, inheriting all functions from the Secretary of State for Housing, Communities and Local Government. Those in turn derive from a 2018 order taking them from the Secretary of State for Communities and Local Government, and so on back in time.
Additionally, the SoS can delegate functions to junior ministers; for example, the 2021 regulations state that they are
Signed by authority of the Secretary of State for Housing, Communities and Local Government
Christopher Pincher
Minister of State
Ministry of Housing, Communities and Local Government
Intra-departmental delegation of functions also requires the agreement of the Prime Minister, although some functions must be performed by the Secretary of State personally. That relates to the so-called "Carltona doctrine", named for a 1943 court case Carltona Ltd v Commissioners of Works [1943] 2 All ER 560, which establishes that a minister is generally responsible for his whole department, and that when some function is conferred on the Secretary of State, it usually doesn't mean that he has to do it himself. Most administrative functions are like that, although the actual laying of secondary legislation before Parliament has to be done by a member of Parliament, i.e. a member of the ministerial team.
So all this doesn't mean that Mr Gove personally decides which flags fall into which category, although observers of his career know that we can't rule that out. But he is responsible for the actions of his department, and the secondary legislation would be laid before Parliament in his name and on his instructions.
Regarding "how and on what considerations" these decisions are made, I have no specific knowledge for flag-related policy. But in general, the secondary legislation is written by civil service lawyers on the basis of government policy, and then approved (or at least not disapproved) by Parliament. For example, after Brexit, the EU flag was removed from the list. The mechanism would be that various people who had never heard of the Town and Country Planning Act became upset about flying of the EU flag, and either wrote to ministers or were Cabinet ministers already. Internally to the department, there would have been a mandate to stop the flag being flown, civil servants would figure out the legal steps, and draft the statutory instrument. In the end we get
a regulation saying
In Class H in Schedule 1, in paragraph (b) of column (1) omit the words "the European Union,".
even though only planning experts would know or care about "Class H in Schedule 1". Similar remarks apply to the companion regulation giving prominence to the Union flag over the Scottish flag, which was a political reaction in Westminster to the SNP-led government in Scotland.