Abbey Wood to Tonbridge: Cross-London allowed?

[DynamicSpirit]

I've just been trying to buy an Abbey Wood to Tonbridge ticket. I was somewhat surprised when tickets.railforums.co.uk offered me an ordinary cheap day return Abbey Wood to Tonbridge and recommended I travel via Elizabeth and Jubilee lines to London Bridge, changing at Canary Wharf: My surprise was because I'm not used to tickets from Abbey Wood to destinations South of London allowing travel by TfL trains: In the past, when buying tickets to places like Winchester or Alton, I've had to specifically buy a Zone 4 to X ticket to be able to do that. But checking on brfares.com confirms that Abbey Wood to Tonbridge allows cross-London travel.

I'm very pleased that this ticket (sensibly) allows use of the Elizabeth line to get into London, but also intrigued: How come some destinations that are South of London (and therefore can strictly be done using National Rail only) allow cross-London travel from Abbey Wood, and other destinations (stupidly, now we have the Elizabeth line) don't? Why the inconsistency? Does anyone know if there's any way to tell which tickets are likely to allow cross-London travel?

 

[paul1609]

For largely now historical reasons Tonbridge is very much a railway ticketing anomaly hotspot. I won't publish them here but it's to do with it having competing routes to London via Sevenoaks and Redhill as well as tickets priced via HS1 and tickets to SE London via the Medway Valley line and Cluster pricing.
If your ticket is routed +not hs1 it will be valid on the route you've described.
People like myself often drive to Tonbridge and use it's cheaper non railway parking and advantageous fares rather than be fleeced by Southeastern on its outrageous Kent Coast fares and country car parking..

 

[Class800]

If the tickets shows Maltese cross symbol you can use it this way

 

[Watershed]

It's to do with the route on the fare - tickets where you're expected to walk between Waterloo East and Waterloo will often be routed "London not Und". Most other tickets where travelling via London Terminals is expected will be routed +via London, thus allowing the use of the Underground/DLR (and according to TfL's policies, the Elizabeth Line).