I second Bianca and Anya.
If you want to avoid on trend names then avoid el start names as these are very popular and become mixed in with Ellie's and Ella's.
Avoid names ending in a or ie or those that start with a vowel.
Therefore underused names to consider:
Rose
Megan
Damaris
Harriet
Meredith
Carol
Janet
June
Tamar
Tegan
Juliet (a mn fave)
Constance
Delphine
Josephine
Louise
Beatrice
Also avoid names that can be confused with its more popular varient. Emmeline/ Emily, Elin/ Ellen, faith/ Day, Everly, Beverley.
Of course if you love a name then go for it no matter how popular!
Dd2 has an El...name but I like it because its unusual but also fits in with her peers.
Other pretty names - Verity, Vera, Veronica, Felicity, Penelope, Tabitha, Wren, Christabel, Polly...