Sophie, Olivia, Isobel, Etta and Darcey are all lovely. My favourite is probably Isobel and least favourite is Olivia but I think they're all good choices.
Other ideas:
Traditional
Anna, Amy, Beatrice, Bethany, Charlotte, Chloe, Daphne, Dorothy, Emma, Emily, Florence, Francesca, Grace, Georgina, Hannah, Harriet, Imogen, Ivy, Juliet, Jessica, Katherine, Katie, Lucy, Lauren, Melissa, Madeleine, Natasha, Natalie, Phoebe, Philippa, Rebecca, Rachel, Sarah, Samantha, Tamara, Thalia, Victoria, Violet, Zoe, Zara
Less Traditional
Astrid, Aria, Bronte, Briar, Cerys, Cleo, Delphi, Devon, Effie, Enya, Fallon, Fenella, Giselle, Genesis, Honey, Halle, Idina, Iris, Jordan, Juniper, Kiki, Kayleigh, Mika, Mala, Nyah, Nia, Ocean, Octavia, Piper, Paige, Quinn, Rivkah, Robyn, Savannah, Saffron, Taryn, Tegan, Verity, Vashti, Wren, Willow, Xanthe, Xanthippe, Yasmin, Zia, Zeryn