Mmm i was busy to add more of those bands to my site, so this is a welcome topic:
Mostly Autumn with ex-Heather Findlay & Olivia Sparnnen are absolute great! The girl on the flute, Ann-Marie Helder (Panic Room, ex-Karnataka) isn't bad either, nice voice. Their very first leadsinger; Heidi Withrop wasn't bad either.
Olivia also sings in Breathing Space a band from the Mostly-keyboardist Iain Jennings (but she'll leave, and maybe Iain got a great opera voice already, fingers crossed).
Magenta with Christina Booth is excellent too, awesome even!
Iona (think Joanna Hogg has one of the most beautiful voices) you already mentioned, check out the guitarist Dave Bainbridge he has made a sole album (Veil of a Gosammer) with some collected female voices...
And their ex-magical Pipe player Troy Donockley has some girls, Barbara Dickson, Joanna & Heather on his too.
Solliste with Emma Brown.
Ezra with Lizzie Buckwell is a fine band too, also from Wales.
Touchstone with Kim Sevoir, check them out.
The Reasoning a promising band from Wales, with Rachel Jones (ex-Karnataka, Ann-Marie sung in that band too).
Landmarq with Tray Hitchings, good stuff!
Glasshammer with Suzie Bogdanowicz.
Kingfisher Sky - Judith Rijnveld
The Gathering - ex Anneke van Griesbergen, nowadays; Silje Wergeland
Skytah - Adriënn Pásztor - vocals,
Kayak - Cindy Oudshoorn
Illumion - Esther Ladiges
Together with Mermaid Kiss, Patoos, White Willow, Krista Johnson (Kara) & Annie Haslam think you've the most well know proggy female fronted bands.
Arjen Lucassen aka Ayreon, with a line of best of vocalist.
Stream of Passion a side project of Arjen, Marcella Bovia (pffff)
Within Tempation (pfoe what a lady; Sharon den Adel)
Epica (- Simone Simons 8,
Delain - Charlotte Wessels (Floor sings some line too)
After Forever - Floor Janssen
Nightwish - with Troy Donockley and a new lead singer called Annete Olson)
Autumn - Marjan Wehlham
I think Nick (from the lovely South) has just about covered all my favourites, with Christina Booth, Ann-Marie Helder, Rachel Cohen, and Judith Rijnveld.
I'd like to add Marta Kniewska, vocalist with Polish band Strawberry Fields, and Holly Ann Thody, vocalist with Bristol band Crimson Sky.