Well only Stevie has had BPI certifications and her list is as follows:
The Wild Heart - Silver. 60,000 copies.
Rock a Little - Gold. 100,000 copies.
The Other Side of the Mirror - Gold. 100,000 copies. (sold over 200,000 copies).
Timespace - Gold. 100,000 copies.
Crystal Visions - Silver. 60,000 copies.
Well that's the list but I know that Bella Donna has sold at least 100,000 copies but for some reason was never certified. Also, I know that Trouble in Shangri-La has sold around 30,000 and I would say the same or probably more for In Your Dreams.
My prediction for her whole catalogue is as follows:
Bella Donna - 150,000.
The Wild Heart - 80,000.
Rock a Little - 120,000.
The Other Side of the Mirror - 220,000.
Timespace - 100,000.
Street Angel - 30,000.
Enchanted - N/A
Trouble in Shangri-La - 30,000.
Crystal Visions - 70,000.
Soundstage Sessions - 10,000.
In Your Dreams - 40,000.
24 Karat Gold - 20,000.
So in total I would say that Stevie has sold around 900,000 albums in the UK. An educated guess but I think it's fairly accurate.