I do not believe for a second that Oliver Cromwell made any distinction between English Rebels, Scottish Rebels or Irish Rebels, he sent them all to the West Indies as slaves if captured.

Right up until the Napoleonic Wars the rule was that once a city was beseiged it was given the chance to surrender. If those inside refused that offer they did so knowing full well that the attacking force, who would now be subject to greater risk and far higher casualties, were free once the city had fallen to exact their revenge on the defenders, so Cromwell was hardly a "Mass Murderer". His troops behaved as any others would have done at the time considering it as being perfectly normal, although I note he did stipulate in his orders that only those under arms were to be killed.

The Head of State of the United Kingdom is precisely that. The King or Queen serves none apart from the people of the United Kingdom of England, Scotland and Northern Ireland and as such can have no such confessor as the Pope in Rome to answer to. The King or Queen is titular Head of the State's Official Church and that is as it should be, that obviously prevents the monarch from being anything other than a Protestant Christian. This arrangement has held England and Scotland clear of the machinations of troublesome Popes throughout history since Henry VIII broke with Rome and established the Church of England, I see no good reason whatsoever to alter that state of affairs.