The Routeing Guide can be found
here.
You are permitted to 'double back' within station Groups.
Section F contains the list of
Group stations.
Code:
GROUP NAME STATION
BANBURY GROUP BANBURY
KINGS SUTTON
So it would take a
negative easement to disallow it.
However it would take a positive easement to allow travel from Kings Sutton itself, via Banbury to stations south/east of Kings Sutton....
Section E contains the
easements.
These ones appear to be relevant to Banbury / Kings Sutton:-
50 Journeys to or from Kings Sutton may double back at Banbury.
...
52 Journeys from stations south of Kings Sutton may not go via Banbury to London nor via Banbury and London to stations beyond London. This prohibition applies in both directions.
So it appears to me that High Wycombe - Oxford is valid via Banbury, as the negative easement applies only to London & beyond.