I made it perfectly clear, I hope, that there is indeed still a prohibition on a Catholic becoming King or Queen.
That does not, however, apply to the office of Prime Minister, which didn’t exist at the time the Act of Settlement was adopted. You appear to be unaware of the provisions of the Catholic Emancipation Act of 1829, which removed the restriction on Catholics holding any public office except that of Lord Chancellor. That exception was removed during Margaret Thatcher’s government.
Had Ian Duncan Smith, a Catholic, remained leader of the Conservative Party, and had gone on to win the 2001 General Election, there would thus have been no impediment to his becoming PM. Had the Catholic Emancipaton Act not been in force, he would never have been elected party leader.
Incidentally, there have been Prime Ministers who have belonged to various Protestant denominatons other than the CoE (Presbyterian, Methodist, Congregationalist). And of course we had a Jewish Prime Minister as long ago as 1868. (Although Disraeli had nominally converted and joiuned the CoE, it now seems to be accepted by istorians that everybody at the time understood and accepted that this was for convenience only). And had Labour won the 2015 election, there would have been a second Jewish PM - Ed Milliband.
>> I made it perfectly clear, I hope, that there is indeed still a prohibition on a Catholic becoming King or Queen. That does not, however, apply to the office of Prime Minister, <<
The Prime Minister’s role is formally little more than chief advisor to the Queen. As the Queen’s advisor, he wields his power indirectly, inasmuch as it he who advises who shall take what cabinet posts in the government. The Roman Catholic Relief Act prohibits a Catholic from taking several advisor roles which partly comprise the traditional office of the Prime Minister:
“It shall not be lawful for any person professing the Roman Catholic religion directly or indirectly to advise his Majesty, or any person or persons holding or exercising the office of guardians of the United Kingdom, or of regent of the United Kingdom, under whatever name, style, or title such office may be constituted, [F1or the lord lieutenant of Ireland], touching or concerning the appointment to or disposal of any office or preferment in the [F2Church of England], or in the Church of Scotland; and if any such person shall offend in the premises he shall, being thereof convicted by due course of law, be deemed guilty of a high misdemeanor, and disabled for ever from holding any office, civil or military, under the Crown.”
This act is partly sensible, given the Queen’s role as head of the Church of England; why should a Catholic advise on episcopal appointments to the Church of England? Yet the inability to fill that role creates a break with the traditional role of the Prime Minister.
Conservative MPs insisted Iain Duncan Smith was incapable of leadership, despite the healthy gains made by conservatives under his leadership. He held a vote of confidence, expecting to win. He lost. (Yes, in part to Betsygate.) It’s hardly axiomatic that his role as leader of the Conservatives meant that he could become PM.
He was said to lack charisma by the party that chose John Major and Therese May, even though he was a Euroskeptic all along.