It's true that I couldn't use the cheaper Southern tickets from London Bridge to travel via Sevenoaks. But the Southeastern tickets are routed Any Permitted so would be valid via Edenbridge or Sevenoaks.
Even if via Edenbridge is not normally a permitted route (which I think it is using maps LB+SD+LK) I believe the any permitted ticket can be used on both routes because i) the Southern trains run through from London Bridge to Tonbridge and ii) the route specific ticket is cheaper than the any permitted (so it would be valid even if changing trains along the way).
But that would be from London Terminals. In this case, the BZ6 -Tonbridge only has 1 fare which is set by SoutEastern trains and routed any permitted. The fare being set by SouthEastern does not mean it can only be used on their services.
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A supplementary question.
I'm using a Kent Rover so one day I'll be travelling from London Bridge to Tonbridge via Redhill and at the end of the day from Sevenoaks to London Bridge and also have a z1-6 non-season travelcard (hence the original question).
On Sunday, I'll be starting at London Bridge and joining the Rover area at Sevenoaks and at the end was planning on travelling from Swanley directly back to Victoria. I'll also have a z1-6 travelcard (non-season). Would a BZ6 to Sevenoaks return ticket allow me to bridge the gap at both ends, starting short at Swanley on the return? So basically is a BZ6-Sevenoaks ticket valid between Sevenoaks and Swanley to reach zone 6 between Swanley and St Mary Cray?
(Obviously, I could actually travel from Swanley to Sevenoaks and not start short but it's extra time and on this particular Sunday 18/08, there seems to be no service between Swanley and Victoria via Bromley South and trains are diverted through Chislehurst).
Sorry to ramble on!