"Fanny' in the British English sense must have been the original meaning of the word - as slang - and then when it was brought to North America we somehow misunderstood exactly what area it was referring to. Since the area it refers to in North America is pretty close.
The problem is that it is a cute slang word, that is less offensive than its many alternatives, in Canada and the USA. Whereas it is definitely not cute wherever British English is spoken. I used to teach English in Asia, and a lot of our girl students would choose "Fanny" as their English name. I had no idea why the British/Irish/South African/Australian/Kiwi teachers would be turning red and laughing whenever they heard the word. With the meaning it has in American English, I would suggest it is not the best choice for a name, but one does hear it; after I found out its English English meaning, I just told my students to choose something else.