I *really* don't like Britain at all but then I live there so it seems strange to me ... have you considered
Britta,
Brigitte or
Brittany? Or even
India,
Indiana,
Vienne,
Paris,
Siena etc if place names are your thing.
Nevertheless I suggest:
Britain
RoseBritain
CatherineBritain
Elizabeth Britain
CharlotteBritain
Elinor Britain
Isabel Britain
JosephineBritain
Joan Britain
MaryBritain
Miranda Britain
Sabrina
I would go for a more traditional feminine name if you are going to use Britain, I would also avoid virtue, flower, unisex and 'word' names :)
alice x