Thanks for additional information, you have some good background material to put together.
I checked the 1881 census and she is not on it so maybe she was in Paris at that time, aged 17. The 1881 census does have a Wincanton family, not many Foyles there apparently, which might be worth a look.
I'd be getting her birth certificate if I were you, at least you know it's right and it will give you a starting point.
In have asked a Luffman researcher if she has anything on Julianna but have not recieved a response yet. Also, I shall ask one of my Foyle correspondents who has done a lot of research for Christopher Foyle, who is of the bookshop family. Christopher has been researching since the mid 1960's I believe.