> Presumably then if the sequence "Mc" is encountered in text with language
> en-UK (or some other code?) it should be collated differently and rendered
> using a superscript c or other method.

The rules are more specific than this. It only applies at the beginning
of a word, and generally only if that word is a surname (family name),
although it will generally always be a surname.  It's a specific rule
for telephone directories, and the like.