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Mynabirds, Nightwish or former Tarja Turunen, those come to my mind, but I wouldn't say Mynabirds are mainstream or Nightwish has a british accent. Hope I helped tho.
Many thanks for the suggestions p310 - neither was exactly what I was after, but I've not heard of Mynabirds before and they sound great, so I'll definitely check them out : )
Thanks again