There one type of tyranny the role of a hereditary constitutional monarch with purely symbolic powers protects against is the tyranny of a [hereditary] monarch with significant or unlimited power...
(As well as the constitutional role of the monarch being designed to protect the British public from monarchs, the continued existence of crown-wearing, palace-dwelling hereditary heads of state probably has a little bit of actual positive influence overseas by reminding some hereditary rulers with actual power that keeping fancy titles and wealth is entirely compatible with allow people to elect representatives to do mundane stuff like passing legislation and running the country)
There's certainly no basis for assuming that protecting minorities has ever been part of the role of the British monarchy though. On the contrary, last time we had a monarch with a deep personal interest in protecting a particular minority (James II, Catholics) we got rid of him.
(As well as the constitutional role of the monarch being designed to protect the British public from monarchs, the continued existence of crown-wearing, palace-dwelling hereditary heads of state probably has a little bit of actual positive influence overseas by reminding some hereditary rulers with actual power that keeping fancy titles and wealth is entirely compatible with allow people to elect representatives to do mundane stuff like passing legislation and running the country)
There's certainly no basis for assuming that protecting minorities has ever been part of the role of the British monarchy though. On the contrary, last time we had a monarch with a deep personal interest in protecting a particular minority (James II, Catholics) we got rid of him.