It isn't that straightforward.
A London to Southampton ticket is valid via a change of train at Havant or at Hilsea, however an easement applies to allow passengers to travel on to Fratton (as Hilsea is an unstaffed station with no disabled access). Travelling on to Portsmouth & Southsea or Portsmouth Harbour falls foul of the 'doubling back' rule unless you are travelling a train that goes from Havant to Cosham via Portsmouth Harbour.
To travel via Portsmouth Harbour would require an additional ticket from Fratton to the Harbour.
I'm afraid I'm going to have to disagree, the routeing guide states that if travel is via a group (in this case it is - Portsmouth Group), then provided the origin or destination of the journey is NOT in the group then you can double back through the group as you wish.
--- old post above --- --- new post below ---
This is the scenario that I really need to think about:
I get on the Waterloo train at Portsmouth Harbour. I have with me a London to Southampton Return ticket. OK, so I might have an additonal ticket to somewhere like Havant or Fratton. At this point, I haven't actually started my journey at Southampton and so I haven't actually broken my return leg. Am I pushing the rules slightly here, or is it ok?
Provided the train you are on is going via Guildford and not Basingstoke, then yes that is fine. The journey can be started at a place short* of the origin printed on the ticket, provided you are on a walk up ticket, and one that permits break of journey on both legs**.
*By this I mean a station on a permitted route between the origin and destination of the ticket.
**This is not an issue here when travelling with SWT. They do not have advance tickets for Waterloo to Portsmouth/Southampton and their Off Peak Returns (as well as all other walk up types) allow break of journey on both legs for these flows.
Southern have advance tickets from Victoria to Portsmouth/Southampton. These do not permit any break of journey, and the journey must be fully completed as per the itenary you are given with the ticket.