Constitutional monarchs exist by the permission of the country’s government, and disappear when that permission is withdrawn. Thus, there is a straightforward remedy for these kinds of things if/when the people’s representatives choose to use it.
> HRH
That's "HMQ". Her children are mostly Her or His Royal Highness. The monarch is Her Majesty.
I raise this not to be petty but to point out a basic error that no taxpayer in the UK would have made, since they would be familiar with HMRC (which collects the taxes), and almost certainly with the initialism HMG.
While sometimes styles are abstruse, the misuse of the most commonly encountered one for the head of state does not support the idea that the misuser knows much about the operation of "her" government or system of law.
There are more substantial things you have wrong.
> pleasure
Not since the Glorious Revolution of 1688 and in particular the Bill of Rights 1689 <https://www.parliament.uk/about/living-heritage/evolutionofp...> and <https://www.legislation.gov.uk/aep/WillandMarSess2/1/2/intro...>.
The assertion by a previous monarch that Parliament met only with his consent was significant in the start of the English Civil War 1642-1651 (and that monarch's death in 1649).
In recent years vigorous litigation, legislation, and debates about how and when ministers may advise the use of the prerogative powers of the monarch to suspend ("prorogation") and dissolve Parliament (for an election to occur) have all been especially clear on the central point that the person wearing the crown has no personal say in these matters; the Queen can only use them as directed by ministers, and only when that direction is lawful (see e.g. the Miller II and Cherry cases 2019 UKSC 41).
> UK's constitution doesn't exist in the way the US constitution does
The U.S.A.'s consolidates several shared aspects into a single codified instrument, but both have significant unconsolidated and even unwritten aspects.
The principal difference isn't in "existence" but in the entrenching scheme wherein the U.S.A.'s federal legislature cannot change certain aspects of itself without the consent of numerous states, whereas there is no such bar on Parliament. Canada's system is a mid-way point: its federal Parliament is able to use the Parliament-alone section 44 amending formula to adjust many aspects of itself and the federal executive, but it must have the consent of one or more provincial legislatures for other changes to parts of the Canadian constitution. The UK Parliament's supremacy means that there is no aspect of the law it cannot change entirely on its own from a legal point of view, however some matters are politically entrenched. It could abolish the Scottish Parliament, for example, from a legal perspective, but with some caveats (see next paragraph). It is unlikely to try to do so, because of the (possibly violent) consequences. Neither Congress nor the Canadian federal Parliament has the legal power unilaterally to abolish a state or provincial legislature.
Canada's constitution is not fully consolidated into the Constitution Act <https://en.wikipedia.org/wiki/List_of_Canadian_constitutiona...>. The UK has almost no consolidation whatsoever, and its Constitutional documents (e.g. the Scotland Act 1998) are in almost all respects just like ordinary Acts of Parliament: the UK Parliament can amend them or repeal them or replace them at will. A Constitutional Act of Parliament, however, will be read by the courts slightly differently than an ordinary Act: in England & Wales an ordinary Act must be explicit when Parliament means it to override the effect of a part of the Constitution (including the non-Act-of-Parliament parts of the Constitution); there are further subtleties in the courts of Scotland and Northern Ireland with respect to Constitutional vs ordinary Acts owing to the history of the formation of what is now the UK. Finally, while the U.S.A.'s Constitution might seem to be fully consolidated, it is clearly not complete, in that Congress retains the Implied Powers (cf. McCulloch v. Maryland) and only controls some aspects of the constitutions of the various states (which differ wildly from one another), elements of which can materially (if typically indirectly) affect the operation of the federal Constitution itself. There is also an abundance of binding case law, and some significant convention (in areas such as the operation of the United States Cabinet, the selection of the Speaker of the House of Representatives and the President pro tem. of the Senate, Senatorial courtesy with respect the formation of the Cabinet, and so forth), which is entirely comparable to case law and conventions in Canada and the United Kingdom.
The comparisons among these three different modern systems can be enlightened by how 17th & 18th century English law was "received" into each system. In the U.S.A., the mechanisms were diverse: <https://en.wikipedia.org/wiki/Reception_statute#United_State...>. These reception instruments are explicitly constitutional instruments (even when not directly in the consolidated constitution) at the state level in a number of states.
> There's really nothing straightforward about the law of the UK
My reaction to that depends on a definition of straightforward. The UK's present constitutional arrangements are not simple, but neither are those of the other two I contrasted it with above. "The law" goes beyond the constitutions though.
There are at least fifty-one different (and one very different) systems of criminal law in the U.S.A.; there are three in the United Kingdom; and there is only one in Canada. There are at least fifty-one different (and one very different) systems of private law (contracts, property law, and other matters where neither the state nor the United States are parties) and administrative law in the U.S.A.; there are about thirteen (one of which is very different from the others) in Canada. There are varying degrees of codification in all of these -- some states and provinces maintain a regularly updated code that is designed to capture the whole of the law (or at least some area of it, like retail sales of goods or the use of roads) from time to time with courts filling in the blanks as necessary, while other states and provinces rely almost entirely on a scattering of different statutes and case law (landlord and tenant law in England & Wales is probably the clearest example).
Since all three of these systems have practical federalism, one can occasionally have to determine the choice of law for a given event. What law governs a contract between someone in Louisiana and someone in Mississippi? Or between someone in England and someone in Northern Ireland? Or someone in Ontario and someone in Québec? This can be straightforward if agreed in advance, and fraught in all three systems otherwise.
Arguably, the more one knows about the law, and the more one draws principled inter-jurisditional comparisons, the more one sees that it isn't straightforward in any jurisdiction. This is probably because there are many ways humans can interact with each other such that they, or another, is unhappy enough to want to seek redress.
Less arguable is the idea that the law of the U.K. is unfathomable to everyone. Indeed, before Brexit the vast majority of cross-border contracts could be expected to have a "choice of governing law" and "choice of jurisdiction" clause favouring English law, even for contracts between parties in Denmark and Germany, or France and Belgium. (See e.g. <https://www.lexology.com/library/detail.aspx?g=c3b07d14-ebad...>). The UK's Technology and Construction Court remains well-regarded after Brexit and has been the venue for resolving disputes among parties who consent to its jurisdiction even if they are in e.g. Romania and Spain. Why? Because the court procedures and legal system in England & Wales are, to these parties (or at least their local legal advisors), more straightforward than many alternatives.