I LOVE Lara and Delphi (I've only ever known one Delphi - it wasn't a nickname.)
I really like Lyra and Reeva (maybe spell it Riva to lessen the Pistorius connection?)
I like Mathilda and Riley (the only girl Riley I know does happen to be American but I don't know if that proves anything).
Florence, Eliza and Tabitha are good solid names but not my particular taste. I have nothing against them though.
Natural fits for Amelia and Theo:
Alice, Alexandra, Bethany, Beatrice, Charlotte, Chloe, Darcey, Eva, Eleanor, Freya, Felicity, Grace, Georgia, Harriet, Hollie, Isla, Imogen, Jessica, Julia, Katie, Lucy, Layla, Madeleine, Olivia, Phoebe, Penny, Ruby, Rosie, Sophie, Scarlett, Willow, Zara
Less mainstream ideas:
Astrid, Aphra, Bronte, Bryony, Calista, Calliope, Danica, Dana, Evadne, Evangeline, Fallon, Fenella, Giselle, Hebe, Harmony, Isolde, Isadora, Jessamy, Jacinta, Keziah, Kendra, Leonie, Larissa, Meredith, Marianne, Nikita, Nyah, Octavia, Paloma, Pandora, Rivkah, Rowena, Rosina, Saffron, Sorrell, Sephora, Talitha, Tassita, Verity, Vanessa, Vivianne, Xanthippe, Yvette, Zeryn, Zipporah