The Forsytes all live in London. I am guessing that the notion of living anywhere else seems to defy their sensibilities, as in, "it just isn't done". Also, living in the country means that you aren't a very easy distance from other family members.

IIRC, Robin Hill is not very far from London. I can't remember which book it was in, but some of the characters visit there for dinner and then take the train back to town, so I always envisioned that it was relatively close by train.