If I remember rightly in the early 2000s the plan if the Queen died was just to redirect every non-news BBC URL to a single page that was just a black background with a centred picture of the Queen and her dates. It was a static HTML file in the root directory of the server. One of the jobs every new year’s eve was to update the numbers underneath the Queen to 1926-200n. Someone with a better memory than me will have the details, and possibly even the files, stored somewhere.
(You’ll need to scroll a bit.)