How academic is she? Epsom College starts at year 9 so she would be starting at the same time as everyone else. There are plenty Girls Day School Trust Schools in the area including Sutton High and Croydon High. Old Palace is worth looking at - ignore the last few years results as they merged with a non selective school for a land grab. Depending on where you choose to live then Reigate Grammar or Woldingham may also be worth considering.
I would second Epsom College. I hear they are looking at changing to Year 7 entry but not in time for Sep 2015 starters.