Augusta - I like this. It's got a 'wise' feeling to it, and also reminds me of summer. Very smart name.
Thomasina - I can't say I like it at all. It sounds to me as though you liked Thomas for a boy, and just stuck an 'ina' on the end when you couldn't come up with anything better for a girl! Not saying this is what happened - just that's what I thought of when I saw it.
Enid - Quite cute - has associations with the children's author which I think is quite nice. I could definitely imagine a little girl with this name.
Rosamund(e) - Beautiful and elegant.
Cecilia - Not bad, but not to my taste. It's always seemed a bit of a weak name, especially with the nn Sissy etc.
Agatha - Not keen on the harsh 'Ag' at the start. Reminds me of hag, rag, sag etc. Just what first pops into my head though, it's a perfectly good name!
Beatrix - Like it a lot - again a children's author name which is sweet. The nn 'Bea' is also nice. A classy name.
Arabel - Not as keen - I think it sounds a bit harsh.
Margot - Makes me think of mangoes, for some reason! I think there are stronger names on your list.
My faves - Rosamund, Beatrix, Augusta and Enid (in that order).
My least faves - Cecilia, Agatha, Arabel, Margot and Thomasina (in that order).