and that's English for you! Let's go up on the downs..

As Ann says, the term is often associated with the south-east of England where they have the North and South Downs.

In literature I've read on Devon c1800, 'downs' is not a term that is much used (hills is a more common term and moors for the large areas of high ground) but on the few occasions I have seen it, it's used to describe hills (or groups of hills) that have flattish or rounded tops. On higher ground, the soil is sometimes not good enough for lush pasture or crops so the tops of hills/downs can be large fields of rough grass (so fewer hedges) or be 'open' (no hedges). I imagine that the lack of hedges and the views from the top would make them good for galloping across.

Marianne and Willoughby said they stayed around the lanes and the others went on the downs. This implies that there were no lanes over the downs here and so presumably they took the carriages up as far as possible on the lanes and then walked, perhaps loading the cold provisions intended for Whitwell onto the horses.