To allow them to do so there is a "legal fiction" where they're appointed to an "office of profit". This disqualifies them from being an MP and triggers a by-election. These are unpaid positions with titles like "Crown Steward and Bailiff of the three Chiltern Hundreds of Stoke, Desborough and Burnham".
https://en.wikipedia.org/wiki/Resignation_from_the_House_of_...
Unless someone can share an instance when an MP could not leave the House?
It also created a curious effect where a Sinn Fein MP (Gerry Adams), who like all Sinn Fein MPs never took his seat because he would not swear allegiance ot the crown, had to accept a crown job to resign, which David Cameron criticised him for (in rather poor form).
And there is nothing aside form precedent preventing a capricious government from stopping MPs from resigning by refusing to make such an appointment.
It's all fun and games having an unwritten messy constitution until a government exceeds its powers to e.g. prorogue parliament against its will and we have to wait months for a court to rule on it when a proper consitution would never have allowed such an act in the first place.
An insurrection like January 6th would suceed in the UK because there is so much arcane bullshit where our constitution should be, we are solely dependent on the good will and character of the current government to run things as they are meant to be run.
It's admittedly a messy state of affairs and many important processes are indeed defined and motivated by convention and tradition only. However, many of these traditions came about whenever the British were forced to deal with events or a constitutional crisis. For example, the monarch does not enter the Commons and Black Rod is admitted only after striking three times exactly because of such events.
January 6th is doomed in the UK for the same reasons it was doomed to fail in the USA: it was based on a flawed understanding how the government really works. The British would have doubtlessly dispensed with tradition and taken expedient measures, and those would become the new Tradition, regardless of whether it is subsequently embossed in Law or not.
Of course the monarchs rather like their position of power and the money they rob (it's only a threat of violence) from the taxpayer and so they don't rock the boat, even when they should (eg when a corrupt, lying PM asks them to please prorogue parliament in order to subvert democracy -- extraordinary prorogation being one of the few reserved powers).
The monarch is perfectly entitled to take a seat in the public gallery of HoC, just as they are entitled to vote, no court would ever deny them such rights as they really derive from shared freedoms given to all UK nationals. When the Tories take us out of ECHR they might lose those rights.
About the only good Mr Alexander Boris de Pfeffle Johnson was to demonstrate how we can no longer trust politicians to be honourable, and must reform our parliamentary system with an assumption that MPs are lying, cheating, thieving beings with utmost contempt for the electorate, the rule of law, and even the institutions of Westminster (buying silence with peerages and stuffing HoL with cronies, for example).
That we cannot, even now, have a general election is a despicable endorsement of the lack of actual democracy afforded to the British people.
Of course the Monarch can enter the visitor gallery, but, as you say, they would enter it as a citizen of the UK, not in their function as monarch.
I'd attribute the deficiencies of the UK's democratic system more to the first-past-the-post voting system, which causes government to be answerable to the winning coalition, not to the whole Parliament. Integrity in Parliament has been going downhill everywhere, not just in the UK.
https://www.theguardian.com/uk-news/series/queens-consent
The king is exempted from racial discrimination laws, for instance, and well into the 1970s it was their official and open policy that black and brown people were unfit for any position more exalted than cleaning the toilets:
https://www.theguardian.com/uk-news/2021/jun/02/buckingham-p...
Everything you think you know about the monarchy in the UK is a carefully crafted fiction by expensive PR consultants, paid by the taxpayers to lobby against their interests.
This is clearly not true, even if they go to great lengths to hide this from the public, in collusion with both UK and Scottish governments. That's before even discussing reserve powers.
All of this is rather shameful, but not surprising. Especially since this happened in the 60s where the attitude of many people was still quite... old-fashioned. Personally, I think actively refusing the Queen's Consent would have caused a scandal not because of the actual discrimination, but because of special rules for the royal household being created, as well as direct interference with Parliament.
This rather demonstrates that the Monarch is aware that they would likely lose out and find their powers further restricted should such meddling turn into an actual confrontation between Parliament and Monarch. The Monarchy exists as long as the public can turn a blind eye to their faults. Yes, that requires that they sweep things under the rug. And this is indeed one of the strongest arguments against the continued existence of the Monarchy.
Wrong, under those theories, Parliament can absolutely alter laws (this requires, of course, royal assent, but the monarch is a component of Parliament not separate from it in theory, even aside from the fact that this function is now essentially ministerial in terms of its performance by the monarch in both practice and constitutional theory.)
You seem to be correct that the monarch is technically not barred from voting (although they don't and shouldn't), but the Lords are not entitled to vote, so I don't think the reason you give is correct.
The US situation can't arise in the UK because they cut the three key roles up differently
In the US the Figurehead and Executive Leader are one man (the US President in this case Donald Trump) in the UK the Figurehead is the Monarch (ie today King Charles) and the Executive Leader is the Prime Minister, who is also Legislative Leader (and historically both these jobs are sometimes done by women). Both countries bizarrely assign multiple jobs with different required skill sets to the same person, which would be understandable if they were tiny nations with a small population but they aren't. However which roles are split this way is crucially different.
The problem on January 6th in the US is that the Executive Leader wants to destroy the Legislature. Yes his Secret Service persuades him not to accompany the insurrectionists, but they're doing what he told them to do.
That can't happen in the UK because the Executive Leader is the leader of the Legislature. If the Legislature don't think he should be leader, they can get rid of him, and not via some "arcane bullshit" but with almost the simplest possible motion,
"That This House Has No Confidence In His [or Her] Majesty's Government".
The "arcane bullshit" is kind of irrelevant in both directions on this question; power grows out of the barrel of a gun. Same applies to the Jan 6th rioters. They seized the building but not the institution. They had no coordinated support network (although it was a spectacular achievement for opportunism) among Congress, or - critically - Trump himself. Nor had they bothered to secure sufficient police/military/paramilitary political support to turn holding a building into holding power.
As to whether it would succeed in the UK: this depends very much on "establishment" support. With that all sorts of things are possible, without it they aren't. The nearest might be https://en.wikipedia.org/wiki/Harold_Wilson_plot_allegations , none of which amounted to more than talk among a few people in a private room. (See fictionalisation of https://en.wikipedia.org/wiki/A_Very_British_Coup ).
Storming Parliament would be met with gunfire in the same way as Jan 6th was unless conspirators had bothered to secure the support of the security services beforehand. Conversely, it doesn't matter what the constitution might say if the security services aren't willing to back it up and decide to fire on demonstrators instead, Bloody Sunday passim.
https://commonslibrary.parliament.uk/decision-of-the-supreme... : as we can see, it was illegal, but not in a way that matters. Parliament agreed to an election, and the public decided to endorse Johnson, the Conservative party, and all prior and subsequent misconduct; the rest is history. https://en.wikipedia.org/wiki/2019_United_Kingdom_general_el...
> The Chancellor may in theory deny an application, although the last time this happened was to Viscount Chelsea in 1842. In the debate over expelling the fugitive James Sadleir in 1856, the Government committed to refusing any potential application he would make.
From the Wikipedia article referenced above.