"Note that these spellings [Dickens' vs Dickens's] are pronounced differently. If you simply add an apostrophe, the pronunciation does not change, but if you add apostrophe 's' ('s), the possessive is pronounced /iz/.
"With singular nouns ending in double 's', as in your examples [princess, boss], Jeff, I think it is more normal to add apostrophe 's' ('s) because the spelling with apostrophe s then indicates the pronunciation required
My comment:
(a) "In these last examples [princess, boss], incidentally, the plural form would have the same pronunciation as the singular:
• 'The princesses's diamonds were worth two million pounds.'
( = more than one princess)
• 'The princesses' diamonds were worth two million pounds.'
( = more than one princess)
Only the spelling or the context would indicate how many princesses there were!"
(b) What BBC -- or Roger -- was saying is: It is impossible to pronounces, by the book, princesses's (the same goes for boss). So "princesses's " and "princesses' " mean the same and pronounced the same (as the latter: princesses').
(2) princess: "Usage notes: Possessive forms: princess's (main form used by academics) The princess's golden hair.; princess' (main form used by newspapers) The princess' golden hair." https://en.wiktionary.org/wiki/princess