Everyone sings them with an "English" accent?

Well, yes - if they're singing them in English. But I've not heard many sung with a "Mary Poppins" English accent. Hugill was from the North West and Killen from the North East, but I don't hear many Yanks singing like a scouser or geordie either.

Hugill also makes the point that many shanties were sung with real or fake Irish accents, and that many ships had multi-national crews: Liverpool-Irish, American Irish and Ireland-Irish crews as well as Swedes, Germans, Negroes.

And I think you'll find that both navies shunned shanties, while both merchant marines used them. If Americans used them more, it was probably that they used smaller crews, so needed more shanties to get the work done.