The only thing that i can't understand is is the reason for it being a US movie. The book is so British! It has mentions of Tiffany Mitchell from Eastenders in it, EE itself, as well as Oxford street, Hyde Park, London in general. In book #1 she basically talks about london throughout, due to her job etc.
So i reckon the movie has big changes to compete with. I still want to see it though, i really like Isla Fisher.