// Incidentally, when was there an RC PM? //
Under the Catholic Relief Act of 1829, “no person professing the Catholic religion” was allowed to advise the monarch on the appointment of Anglican bishops. Given the PM's role in the Church of England, having a catholic PM could have been awkward. this was not repealed until 1983. Boris Johnson is catholic, but he didn't "come out" until after he'd assumed office. Tony Blair too is catholic but he'd left office by the time this was made public.