Waterloo is a member of the London Group. Horsham is a routeing point.
http://www.atoc.org/clientfiles/File/RSPDocuments/routeing_point_identifier.pdf
Routeing permissions for London Group-Horsham are: LB, LP, PD+SC, SC
http://www.atoc.org/clientfiles/File/RSPDocuments/permitted_route_identifier.pdf page 572
Maps PD+SC allow, amongst others, Waterloo-Clapham Jn-Wimbledon-Surbiton-Woking-Guildford-Eddingham Jn-Leatherhead-Epsom-Dorking-Horsham
http://www.atoc.org/clientfiles/File/RSPDocuments/maps.pdf pages 81 & 88
But as Epsom is north of Leatherhead, this route would be doubling back through Leatherhead and Ashtead, which is not allowed unless specifically permitted by the group station rule or an easement. Epsom is just a routeing point, not a group, so the group station rule does not apply, and a search of the easements
http://www.atoc.org/clientfiles/File/RSPDocuments/easements.pdf shows no easements involving Epsom. Hence the doubling back is not allowed, so this route is restricted to:
Waterloo-Clapham Jn-Wimbledon-Surbiton-Woking-Guildford-Eddingham Jn-Leatherhead-Epsom-Dorking-Horsham.
Therefore your season covers you Waterloo-Woking. The National Rail conditions of carriage says:
"19. Using a combination of tickets
You may use two or more tickets for one journey as long as together they cover the entire
journey and one of the following applies:
(a) they are both Zonal Tickets (unless special conditions prohibit their use);
(b) the train you are in calls at a station where you change from one
ticket to another; or
(c) one of the tickets is a Season Ticket (which for this purpose does not include
Season Tickets or travel passes issued on behalf of a passenger transport
executive or local authority) or a leisure travel pass, and the other ticket(s) is/are not."
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf
Your season covers you Waterloo-Woking, and you would buy a normal ticket Woking-Southampton. You would be covered by part c as you have one season ticket and one ticket which is not a season, so you could travel on a train not stopping at Woking.