Why say "Leftenant" and not "Lootenant"?

Dear All,
just been asked a question by a civvy work colleague as to why the British say "Leftenant" and not "Lootenant" as per the French and US?
Must admit, it has never crossed my mind and I do not know. Was there a specific time/reason why we added an "F" to the spoken word?
Funny. I was once told by an english teacher, it had to do with the way lieutenant was written on a commision historically, with a latin V being used instead of U.

So people would sometimes read it as Lievtenant and pronounce it "leff," which stuck.