Another difference: Amanda's character.
In the opening novellas, Amanda is a fully independent young woman who works as an art appraiser. She has a very clear idea of what she wants out of life or, rather, what she doesn't want. She comes across as a kind of British Isabella Stewart Gardner--equally straightforward and blunt.
She also is a city-girl, all the way.
The Amanda character in the television series is the victim of 1950s assumptions about a woman's role blah blah blah. She comes from a wealthy family. She can't decide what direction to go in life. She is, frankly, a (broody) stereotype of a 1950s woman.
My mother was a product of WWII and 1950s thinking, and in some ways, she echoed those sentiments. And in some ways, she totally didn't. In sum, Runcie's version of Amanda is more human than the series' version.
It is possible that the television series made Amanda some kind of victim of cultural expectations to MAKE A POINT. But I suggest an entirely different reason.As mentioned above, Amanda in the book is a city girl. In the series, she suddenly becomes nostalgically fond of the country. I suggest that according to modern mores--which can be as powerful as those of the 1950s and earlier--a heroine who is NOT fond of trees and ponds and grass and furry creatures is not..."the done thing."
That is, a "good" heroine will pine for the days when she could run about the fields and have picnics and jump in rivers...rather than the days when she could visit museums and window-shop and ride buses.
Nostalgia is connected to green stuff.
It isn't in reality, of course. It's connected to lots of different things. But certain tropes are hard to shake. Shorthand for "this character is unhappy" becomes "she wishes she could get back to nature."
I comment elsewhere that in the past, nature was the thing that would kill you. Now, it is the thing you are supposed to crave.