Hi Michael,
Please can you tell me the times of direct trains tomorrow (31st Dec)?
I know that engineering work often sends trains this way, but it is NOT a regular service pattern. If the fare finder offered zone 2 only fares by default then people would be asking how to make the journey without being charged extra when there are no direct trains (ie most of the time).
If the line via Primrose Hill ever becomes a normal 7-day a week service then I would expect fares to be created. There is nothing to stop you contacting TfL and asking them to consider adding direct fares, but in my opinion this is unlikely to be done in the current setup.