So the question would be, in the event of (say) a monarch converting, which would be the path of least resistance for Parliament: Changing the Act of Settlement, or changing which particular figurehead sits in the Irrelevant Chair?
I’m not sure about that. The monarch is the Supreme Governor of the C of E, but that doesn’t mean he has dictatorial powers in it. The C of E has a legislative process for adopting pieces of legislation called “measures”. They are voted on by the Church’s general synod, but after that vote they are submitted for final approval by Parliament. That’s necessary because in British constitutional law, C of E Measures have the same force as an Act of Parliament - a consequence of the status as the established religion. (Of course, as far as intra-ecclesiastical affairs are concerned, Parliament would be highly unlikely to refuse to approve a Measure adopted by the synod, but that’s a political matter, not a legal one). I would guess that reuniting the C of E with the Church of Rome would require amending or abrogating some existing Measures, which the King could not do on his own, just like he can’t legislate on his own to abrogate or amend an Act of Parliament.
Quite.
Absolutely the former, it would pretty uncontroversial nowadays. Whereas a few centuries ago the idea that a monarch might be Catholic led directly to the execution of one king and the deposing of another. Nowadays it would barely raise a shrug.
I’m sure the government would find it a nice break from actual meaningful legislation that is hard to pass.
Put me in the “we won’t know until we know” category. Never underestimate people’s capacity for petty bigotry.
On the one hand, it’s ridiculous. On the other, so is the monarchy.
Come to think of it, the answer would probably depend. If a monarch were to convert to Catholicism because they did’t want to be monarch any more, then Parliament would probably let them quit and name the new monarch, because no good can come of a king who doesn’t want to be. And if the monarch were to convert for reasons of personal faith, then they’d probably notify Parliament in advance of the official conversion, so they’d have plenty of time to change the law beforehand (even from the Catholic Church’s own procedures, converting to Catholicism isn’t a hasty process).
I should add it would be greeted by a shrug in almost all the UK. In Northern Ireland (and a few parts of Scotland) it would be moderately controversial. But even there the actual Catholic religion has long since become insignificant other than a marker of which community you belong to. No one is going assume the king is going to start supporting sein Fein (or Glasgow Celtic) if he converted to Catholicism
I’d say even Farage et al, would have trouble whipping his gammon followers into a frenzy over it. Maybe if (as Charlie has mentioned he’s interested in) he changed his title to “defender of faith” from “defender of THE faith” then he could whip up some outrage about the royalty going woke (and how the real aim is to appease all the Muslim refugees). But just converting to Catholicism, it’s a big “meh whatever”.
Who?
Charles I was a staunch C of E episcoplian. The religious criticism against him came from Presbyterians and Congregationalists.
Sure, but his marriage to a Catholic certainly didn’t help assuage that criticism.
He had a Catholic wife and was persistently accused of crypto-catholism, particularly when he did things like bring troops from Ireland (even though the troops in question weren’t Catholic they were in the mind of his opponents). He wasn’t Catholic but the idea that he might be was a big contributing factor to opposition against him.
I think he was also friendlier with the Spanish than many English liked.
Foreign policy meant juggling just how close or distant you could to be in relation to the dominant power(s) in Europe (still does). In those days that was France and Spain, both Catholic, but rivals. Charles’s father began by seeking rapprochement with Spain, but it was a fractious relationship and Charles didn’t help by sneaking off unannounced to Madrid to negotiate a Spanish marriage on his own initiative (complete failure, of course). The pendulum swung back and a French marriage was arranged. France at that time was more tolerant of Protestants, but by the end of the century it was Louis XIV’s reversal of that policy that encouraged suspicions about James VII/II’s policy of toleration for Catholics, hence 1688 and all that, and a century and a half of hostility with France.
You’re forgetting the third dominant power in Europe at the time - the Dutch, who, being Protestant, were considered the natural allies of England, and the nemesis of the Spanish. Cozying up with Spain meant betraying the Netherlands. Of course, after the Restoration on the balance shifted and France became the Dutch enemy, leading to the Anglo-Dutch wars and the Dutch conquest of England.
I was under the impression, probably wrongly, that the Netherlands wasn’t that dominant, whatever ideological sympathies there may have been, until William had taken over from the republicans and stood up to Louis XIV. Economic/trade rivalries had sparked an Anglo-Dutch war under the Commonwealth, which you would have thought more ideologically aligned with the Dutch republicans.
I think rather than being too close to either france or Spain, he played the game of great power politics with shifting alliances with France and Spain (as all English monarchs were expected to do) but did it very badly.
The kings of France and Spain played him like a fiddle (at one point he ended up, allied with, and giving naval support to France during their civil war, against the protestant hugenots he was meant to be going to war to support) And when he did go to war against the Catholic powers it was a complete fiasco.
Was all that because he was an incompetent ruler in way out of his depth? Or was it because he had been secretly converted by his devious papist wife and was now working for the pope? Just asking questions…
Religion was inextricably linked to international politics. After all, for Henry VIII part of the problem was that the pope was allegedly controlled by the Spanish. A good analogy might be how communism in the 20th century was seen in the West not just as a desire to radically overthrow the current order, but also that communist parties were esentially tools and proxies of Russia and later also China. Communist sympathies implied one was willing to betray your country to the USSR.
Today, how many Brits care or worry about French or Spanish domination?
Enough to pass Brexit? 
More seriously, at bottom it’s all xenophobia, whether of individual people worrying about jobs and culture or of governments worrying about power and influence.
That’s true, but it’s only part the reason for anti-catholic sentiments in Britain.
The actual theological and ceremonial differences between Catholicism and Protestantism (and between the different types of Protestantism) used to really matter to people. The church reforms that Charles I tried to push through and led directly to the civil war seem utterly trivial and inconsequential to us today, even the most conservative Christian would not get upset over them. But at the time ordinary people were literally up in arms over them.
That’s why it would be an absolute non issue if the monarch decided to convert nowadays. The actual religion itself has become pretty much insignificant even in parts of the UK where there are still Catholic-Protestant tensions. Those tensions are actually just between different communities, Catholic and protestant are just labels. No one actually cares if someone believes in the doctrine of consubstantiation or not
Let’s say The Brits and The Pope agree that Anglicanism is really Roman Catholicism Lite. There are a lot of reasons for the Irish to hate the English, but if they were now the “same” religion, would that help the relations between Eire and Ulster?