Jay, since you ask about Jo as a boy’s name in Britain, here’s my two cents, or tuppence-ha’penny: It’s a class thing. If you’re a horny-handed son of the sod, you get abbreviated to “Joe”. But that has too much of the whiff of the cloth cap about it if you’re higher up the social scale, so they go a stage further and become “Jo”. The longtime Liberal Paty leader, Jo Grimond, was an Old Etonian, like Jo Johnson. Tim Rice, lyricist of The Lion King, Evita and Joseph and the Amazing Technicolor Dreamcoat*, has a brother called Jo Rice, who teamed up with Tim to write a series of books on the British hit parade, very useful for finding out which Pat Boone record got to Number 37 in 1963. I think Tim and Jo went to Lancing.
An exception that may or may not prove the rule is Jo, Dickens’ grimy road-sweeping urchin in Bleak House. So I could be all wrong. It’s still early and I haven’t had my first cup o’ Jo.
(*known as Jo and the Amazing Technicolor Dreamcoat to his friends)