There are Off Peak Returns (formerly Saver Returns - SVR) for journeys to/from London available from stations in the former Network SouthEast area. I don't think there is any hard or fast rule as to where these are or aren't offered. A cursory look at a few fares suggests that the closer you are to London, the less likely it will be that there is an Off Peak Return (SVR) available.
Having investigated in depth a few years ago, the distance from London when SVRs become available seems to be about 35-40 miles. There are a few exceptions, I think the main airports are the main examples.
Paul.