Kate Winslet, Helena Bonham Carter all did lots and lots of period films at the start its how you get big look at them now both have good carrers. Trust me keira is doing well some of the stuff that shes got coming up is different if she did just nowadays films (which lots of actors do like cameron daiz etc.. no one have a go at them for always doing now adays films do they) then she might not be as sugessful.
Look kate Blanset or where ever her name is she moved to LA and hasn't done well the same with most others which i probably don't known coz there haven't done well, keira was the only one on forbes highest paid list this year that wasn't an american (might not actually made that much but Will Smith, camron daiz proably did get as much as frobes said too just don't want to admit it).
The british film buisness isn't that big and period flims are the only ones that made big hits as there easier to come up with, americans might find british comedys as good as they don't always get the jokes or are cuture most british T.V Shows (like the office) have been americanized (which is sad as we have to learn about there slag words and culture from 'Heroes' 'Ugly Betty' etc...)
we haven't had a big hit comdey in years are last was "love actually" which keira was in if there was another like that around i better kiera would be in it.