Well, predicting that if the monarch does X a revolution will follow is not treason felony, even if the prediction is offered to the monarch himself. A monarch needs advisers who will point out to him the likely or possible consequences of his actions; providing that advice is not treason.
But, to be clear, it’s no answer to my point to say that the events which would follow if the monarch withheld assent from a Bill enacted by Parliament and supported by the government would or might be illegal. I’ve already said that the events might include one or more revolutionary acts. So what? “X would be illegal” does not mean “X cannot or would not happen”; illegal things happen all the time. The status and/or powers of the UK monarch have been constrained — permanently constrained — by illegal acts in the past, both in the UK and — as US boardies must surely know — in other British possessions. This can happen again. And it need not involve war, or the use of force.
Except that’s not what I said. The PM directly threatens that they will depose or otherwise take away power if he does not RA the bill. Such a direct threat is, I believe, a treason felony.
The scenario I outlined did not involve the PM threatening such a thing. In my scenario, the PM’s response to the monarch’s rejection of advice to assent to the Bill was to resign. He might also - constitionally, he should also - point out the likely downstream political consequences; that would not be treason felony either.
(The bottom line here is that you can’t serve as PM unless you enjoy the confidence of the House of Commons. Because, without that, you don’t have any money. Or an army. And, equally, you can’t serve as PM unless the monarch accepts your advice. Because, without that, you haven’t got any legal authority.
Put the two together and the bottom line is the that king can only appoint a PM whose advice he will accept, and a PM will never advise the king to go against the will of the House of Commons. So if the king refuses, against ministerial advice, to assent to a Bill , he doesn’t have a PM, and as long as he persists in refusal of assent he can’t appoint a PM who enjoys the confidence of the Commons. And he can’t govern without a PM because he, too, doesn’t have any money, or an army.
So, in this scenario the UK constituion is fundamentally broken. It won’t work any more.
Where we go from there almost certainly involves some revolutionary acts, as attempts are made to re-establish a functioning state. And, whlle the exact course of events is unpredictable, the likelihood that the functioning state that would emerge from this revolution would be an autocratic monarchy is - ahem - not high.)
One point to remember is that, if a monarch refuses assent to a bill, they have some reason why. There’s something about that bill that the monarch considers absolutely unconscionable. And whatever that is, most likely the monarch wouldn’t be the only person to hold that view. Is their view shared by 10% of the population? 20%? 49%? 55% and only a quirk of districting put the other party into power? Keep that in mind, in any hypothetical of how the resulting revolution would proceed.
I’m thinking, most likely, a bloodless revolution. The institutions of the dysfunctional state will still exist, and will still be capable of action, albeit revolutionary action (because their actions are no longer a constitutional exercise of the authoirity of the crown-in-parliament). So they’ll put in place measures that they think will work, practically speaking, and that includes being at least tolerated by the bulk of the population. They’ll also put in place measures which, so far as possible, do not imperil their own situation as key institutional elements of the state. (Which, among other things, the measures almost certainly will not include accepting the veto of a Bill enacted by Parliament.) And then they’ll try to retrospectively construct a legitimacy for them afterwards, most likely by having the arrangements endorsed or ratified through a popular referendum, or through formal acceptance and confirmation by a reduced and chastened monarch, or maybe both.
Possibly the new legitimacy will involve some fictional recharacterisation of the preceding events. (“The king didn’t really withhold assent from the Bill. Due to the pressure of events and his own mental health challenges he wasn’t fully competent, but this hadn’t been recognised in time and the activation of the regency provisions had not gone through. So in reality there never was a refusal of assent! And, happily, His Majesty is now fully recovered. God save the King!”)
Or, it could involve an explicitly revolutionary constitutional re-ordering. (“The source of all legal and constitutional authority in this country is no longer the crown; it is the people. The people themselves have asserted this by enacting this shiny new constution by referendum, under which the king continues to be the head of state but he derives his authority from the people, through this constitution. The king, and all the other officers of state and members of the legislature, have sworn an oath recognising and accepting the constitution enacted by the people and committing themselves to observe, uphold and defend it — or, those who haven’t felt able to swear that oath are no longer in office. God save the King!”)
They can adopt an ex post indemnity act once the new regime (with a different king, or no king at all) is in place, stating that nothing that occurred during the transition from the old regime to the new one constitutes treason. The precedent for this would be the measures taken to restore the monarchy in 1660. For the time of the transition itself (which can be very short), the courts would refrain from intervening with treason trials; it’d all be way too political.
My inspiration was in fact the 1937 Constitution of Ireland, which was enacted by the people even though under the then constitutional order the people had no right or power to do such a thing. In enacting the constitution, the people asserted a new fundamental norm as the ultimate ground for legal validity. The then officers of state — judges, legislators, etc — all took an oath accepting this. By these means a revolution was effectively accomplished. Since then all legislative, executive and judicial power exercised by the various institutions of the state has been seen as having been conferred on them by the people.