Your day’s travel can be completely covered by:
8.10 for a 1-2 peak cap (also covers all bus travel)
4.40 for a 3-6 Thru extension
Makes 12.50
The extension fare doesn’t _need_ to exactly match the fare for a similar zones+farescale _journey_, but in this case it does – for example, a journey from Pontoon Dock to Erith, which is a Thru 3-6 journey, also costs 4.40 peak.
Even though your rail journey went 6-5-4-3-2-1-2-3-4-5, just one extension fare for 3-6 covers you on the way in _and_ the way out.