This novel moves between the town and the country, just as most people of a certain class did in Austen's time. Austen's very concerned with the social milieu of her characters, and her settings very clearly reflect this concern. Barton is the countryside, and our main characters live in the shadow of the landed gentry (the Middletons at Barton Park). It's notable that Elinor and Marianne are always on the outskirts of the people who move back and forth between London and Devonshire – they themselves aren't well off, and they are dependent upon these wealthier connections to stay in the social loop of middling-to-high society.
Austen's particular moment in history is an interesting one. Her novels are fascinated with the odd and peculiar interactions between people of different social levels – a relatively new development in England at the time. Her books are firmly planted in the bourgeoisie, a class that was on the up-and-up as she wrote. None of her characters in Sense and Sensibility are firmly planted in the upper echelons of high society; even the wealthiest ones aren't integrated into aristocratic social circles. Rather, the spaces in which the Dashwoods move are those of the middle classes (and believe us, there are about a million different levels of "middle class" operating here).