Agreed with Dan_ on Niki's choice, that'd suit her.
Beverley - Missing You by Tina Turner
Same Difference - What Can I Do? by the Corrs would be interesting
Maybe Heartbeat by Steps.
Hope - Too Lost In You would be very good for them. But I can see it being Goodbye, or Viva Forever, by the Spice Girls (the latter being a song they really want to do).
Rhydian - Your Song by Elton John?
Leon - Maybe "Iris" by the Googoo Dolls.