Why is it pretentious..?
Its an Old English pronounciation... and he's English ..
Ralph says this:
Why is your named pronounced "Rafe"?

"It's a rather outmoded use of that name in English. I was named after my stepgrandfastep great-grandfather. No sorry, step grandfather, who was called "Rafe" and spelled his name Ralph. And my father admired it and he wanted to call his son "Rafe" and spell it Ralph. I've met the odd "Rafe-but-Ralph." They do exist, but they're quite rare" 

http://www.newsday.com/entertainment/movies/nyc-fiennes,0,50909.story?coll=ny-moviereview-headlines