Serena - I like this one. I went to school with a girl named Serena who was an absolute hoot, so there's positive associations for me. But I think it's a nice name, very underused etc. I suppose it does run the risk of people automatically thinking of Serena Williams, but I hardly think one of the world's greatest athletes being an association is a bad thing!
Apolline - I'm not a fan of this. I can envision a lot of spelling and pronunciation issues.
Celestina - Not a fan of this either. I don't mind Celeste (although, ever since I heard someone tell them it reminded them of the word moleste
I've gone off it a bit), but Celestina is a bit OTT. Valentina might appeal to you, it's quite similar in sound?
Cressida - Bonas is the only person who comes to mind, and I'm not even in the UK. 
Iris - I like this one, but it's definitely more popular than the rest, if that's something you're concerned about!