The Penguin Book of English Folk Songs (early edition) served us well way back - revisiting it recently for notes convinced me that it remains the best overall introduction to the English repertoire ever produced (Abe Uk Books have used copies from around £4).
Don't know about the ukulele - or any musical accompaniment - I'm not against it, (I sing accompanied when I get the opportunity) but I think that the best way to build a new repertoire is to try the the songs unaccompanied first and then decide if they need accompaniment - most of them don't - the ukulele (or any short-duration instrument) presents far too many problems for anybody starting build a repertoire.
Wish her luck.
Jim Carroll