Not sure where to place this - but as it is about how much is permitted for those in line to the throne, I figured this might be an appropriate place (as the discussion above will not be revived given that Charles now is king):
I am amazed at what some royals (spares) can get away with from a government point of view - as they are still in line to the throne even though some of them have decided (or it has been decided for them) to fully live their own lives - and in doing so sometimes damage the monarchy.
Is there no such thing as 'ministerial responsibility' in other countries? In the Netherlands, the ministerial responsibility applies to all members of the royal house (previously that was equal to those in line to the throne and their spouses - which makes most sense to me-; now the king's nieces and nephew although still in line to the throne are not included). Meaning that those closest in line to the throne (in practice children and siblings of monarchs - if they remain in line to the throne) cannot just do and say as they please because the prime minister bears responsibility for what they do and say and will need to answer for their actions in parliament if push comes to shove. To me this makes sense given that each of them could become the future monarch (or spouse) and therefore should be 'above reproach' and their actions reflect on the Monarch and monarchy (by virtue of them not only being close family members but eligible to become the future monarch; in most cases that is only one terrible accident away).
I don't know how it works in the Netherlands, but ministerial responsibility normally applies to
official acts of the monarch that are carried out on ministerial advice. For example, the ministers bear responsibility for any order approved by the monarch in a meeting of the Privy Council or, generally, for any appointment of a public officer made by the monarch, or for a royal proclamation dissolving or proroguing Parliament, or declaring war.
By extension, I suppose ministerial responsibility also applies to official acts undertaken by the heir and other members of the RF on behalf of the King. So, for example, if William is on a official visit representing the UK and gives a speech in connection with that visit, I understand that falls under ministerial responsibility too.
Ministerial responsibility does not extend in my opinion, however, to the royals' private lives. Furthermore, as Harry and Meghan are no longer working royals, and no longer officially represent the United Kingdom, nor act on the advice of British ministers, they are personally responsible for whatever they say or do.
I would suggest that including parliament in decisions on who remains in line to the throne upon marriage would be a small step that might mitigate some of the issues (although some issues arise after marriage); as it seems most monarchs nowadays are rather reluctant to withhold (official) permission from their close family member to marry - even if they would marry someone who is deemed fully unsuitable by everyone else. In that case being able to blame parliament (or deciding not to ask permission given that the outcome might be guaranteed) would be a way to diffuse such a situation.
Parliament is not directly involved, but the government is since the King's consent to a marriage of one of the first six persons in line is declared in the Privy Council, so, in my opinion, it falls under ministerial responsibility too.
Involving the King and the government seems to be the standard approach as far as royal marriages are concerned. It is, in one form or another, what is done in practice in the UK, Denmark, Belgium, and, more explicitly, Sweden. The Netherlands is the only European kingdom where an act of Parliament is required to consent to a marriage of someone in the line of succession. Spain, on the other hand, is a bit of an outlier in the sense that consent is not required, but, in order to effect a prohibition of the marriage that may affect the line of succession, both the King and the Parliament have to be involved.
