If we're thinking Tori Amos, don't forget the original - Kate Bush. The girl recorded "Wuthering Heights" aged 16, meaning she wrote it aged 14 or 15. Now that's pretty amazing, if you ask me.
I'd second Joan Armatrading. Saw her a few months back, and she was great. It seems to be unusual for women to have such big stage presence without doing much, but JA has it in spades. She also rocks as a guitarist.
Beverley Knight and Duffy, from more recent soul stuff. Both have great voices. I'd like to see BK doing more - she's not had a new CD out for a while now.
A couple of relatively unknown English singers. Jane Taylor is a fantastic singer and writer, both live and recorded. Abbie Lathe is a bit ho-hum recorded but great live, especially with some of the covers that her band does.
And very locally, Jacqui Carnall from Cambridge is pretty bloody good and deserves recognition.