By the requirements of the story, the character spends half her on-screen time slightly insane, and that can be heard in her speech even when she's coherent.
Here are the voice samples of the four actresses who are canidates for the role of the character (who presently needs a voice)
Gina Bellman: Claire Jackman, Jekyll.
Beth Riesgraf: Parker, 0:50-1:08, Leverage.
Michelle Ryan: Catherine Reimer, 16:55-17:22, Jekyll.
Indira Varma: Niobe, Rome.
I considered adding Naoko Mori to the list of canidates, but I only know her in two roles (Torchwood and a tiny bit of Absolutely Fabulous, so I don't know how she sounds when portraying insanity/hysteria or rage.