"protect the liberty of individual citizens"? Where do you get that? The sovereign's function is to legitimitise her governments. In turn, Parlaiment ensures the legitimacy of the sovereign. If any liberties are ensured or curtailed, it is the government, not the sovereign to do it. The only words to come out of the mouth of the monarch are written by the government. Britain may not have a single written constitution, but it does have one- in the form of laws passed by Parlaiment and ensured by the courts.
The Queen has done an admirable job in her 55 years on the throne, and I certainly wish her more years, but I am not at all hopefull for a future reign by the repbrobates to come. Unless their is a massive show of mourning for the loss of a British icon, I think a collapse of support for the monarchy itself may come about.