Thanks for the replies. I would probably have used a Southern 'Daysave' ticket if I were not making this journey on a Sunday, when Southern do not operate any services out of Southampton. (I am planning on using a Daysave to get there two days earlier, tied in with doing various units and lines during they day.) I would presume this £13.40 single will be valid on Gatwick Express between Gatwick and Victoria? (It will allow me to scratch in my first 460!)
Yes the £13.40 single is valid on Gatwick Express but crucially only if you travel via Brighton and then the Gatwick Express you catch, must go via Redhill.
The only route valid on GE is as such- from Southampton then Havant (also via Portsmouth), Brighton (even Lewes), Gatwick, Redhill and then either via Selhurst or Crystal Palace to Victoria- on maps CW & LB.
Rather weirdly, the journeys via Horsham only allow you to travel to London via Dorking and not via Gatwick Airport.
The other sets of maps for Southampton Gp to London Gp are:
CW+PD- Allows travel Southampton to Havant (also Portsmouth), Guildford then Waterloo (via Woking and Oxshott)
CW+PD+SC- Allows travel Southampton to Havant (also via Portsmouth), Guildford, Epsom then Victoria, Charing Cross, Waterloo, Blackfriars or London Bridge (via a variety of South Central Metro routes)
CW+SC- Allows travel Southampton to Havant (also via Portsmouth), Ford, Horsham, Dorking, Epsom then to Victoria, Charing Cross, Waterloo, Blackfriars or London Bridge (via a variety of South Central Metro routes)
MW+WR- Allows travel Southampton to Eastleigh (also via Fareham), Basingstoke, Reading then to Paddington
MW+WX- Allows travel Southampton to Eastleigh (also via Fareham), Basingstoke, Farnborough, Woking, Surbiton, Clapham Junction then to Waterloo or Victoria
WX- Allows travel Southampton to Eastleigh, Basingstoke, Farnborough, Woking, Surbiton, Clapham Junction then to Waterloo or Victoria
The Daysave by the way would have been only dubiously valid (if you had wished to use it on your journey south) with a previous discussion not coming to a conclusion either way (
http://railforums.co.uk/showthread.php?t=22656&highlight=daysave)