The argument isn't about what is and what is not a title, it's whether titles in the British-English Rockbox language file should use title case.
I think that is actually the important question. Or at least, that and "In the UK is it 'Harry Potter and the half-blood prince' or 'Harry Potter and the Half-Blood Prince'?" If they are titles, then title case is appropriate if title case is used by the language for titles.
