For the Bath-Swindon ticket, you are valid by:
a) direct trains,
b) the shortest route. In this case, it is obvious that this is the route taken by the direct trains, certainly not via Salisbury,
c) other routes permitted by the National Routeing Guide:
Bath Spa is a routeing point.
http://www.atoc.org/clientfiles/File/routeing_point_identifier.pdf page 5
Swindon is a routeing point.
http://www.atoc.org/clientfiles/File/routeing_point_identifier.pdf page 59
The only routeing permission for Bath Spa - Swindon is: BL
http://www.atoc.org/clientfiles/File/permitted_route_identifier.pdf page 62
Map BL allows:
Bath Spa - Chippenham - Swindon
http://www.atoc.org/clientfiles/File/Maps.pdf page 8
For the Swindon-Farnborough ticket, you are valid by:
a) direct trains,
b) the shortest route. I suspect this is via Reading to Farnborough North, any route via Salisbury would certainly be more than 3 miles longer than this/
c) other routes permitted by the National Routeing Guide.
Swindon is a routeing point.
http://www.atoc.org/clientfiles/File/routeing_point_identifier.pdf page 59
Both Farnborough stations are a member of the Farnborough Group.
http://www.atoc.org/clientfiles/File/routeing_point_identifier.pdf page 23
The only routeing permissions for Swindon-Farnborough Group is: WR+MW
http://www.atoc.org/clientfiles/File/permitted_route_identifier.pdf page 986
Thus, the only mapped route I can see for this ticket is:
WR: Swindon-Didcot-Reading p119
MW: Reading-Wokingham-Farnborough p78
http://www.atoc.org/clientfiles/File/Maps.pdf
Thus, neither of your tickets would be valid via Swindon. To use this combination, you should travel:
Bath Spa - Chippenham - Swindon - Didcot - Reading - Workingham - Farnborough North. As your Bath-Swindon ticket is a season, under condition 19(c), there is no compulsion to travel on a train which calls at Swindon.