Final authority must lie somewhere. In the US, it is theoretically in the Constitution. Because the Constition is an actual document it means that the authority it gives to government is limited by its terms. It means that the government cannot easily enact forward-looking legislation because there will always be somebody to scream "That's unconstitutional!". On the other hand, the fact that it is an 18th Century document means that it is easy to enact regressive legislation. It also means, as we have seen, that the decision of a few bigots, empowered by said document, is able to strip significant human rights from half the population at a stroke, leaving the Federal government helpless to take remedial action. Effectively, whatever measures the people want enacted, whatever and whoever they vote for, the actions of the government remain limited by the Constiution and its outdated principles and provisions.
In the UK, the Crown (not the monarch) is the source of authority. Parliament is not sovereign, but is granted authority by the Crown to enact a mandate given it by the people. The authority of the Crown is abstract and apolitical - it has no rules and provisions. Parliament alone has the right to legislate. The Civil Service, the judiciary, the police and the armed forces owe their loyalty and authority to the Crown. This gives them the right and duty to disobey orders from the Prime Minister or any government offical if these are not approved by Parliament as a whole. It means that the Courts are entitled to strike down any law or regulation they deem unjust, giving ther people a second avenue to challenge the government.
Theoretically, the monarch could live in a suburb and do an ordinary job most of the time. The only legal requirements of the role are to open and dissolve Parliament, sign off on Acts and ask the majority leader in Parliament to form a government after an election. All the rest of the 'BS' as you call it is either traditional or voluntary and usually both. Look to see some reductions in future. The King has already vetoed a formal Investiture for the new Prince of Wales on the grounds of cost. I also supect that any Coronation will be a more subdued affair than that of the late Queen. The King has frequently spoken about 'slimming down' the monarchy to reduce expenses, and I think he's going to do it, though how radically is another matter.
