Why not simply buy a paper travelcard?
If I had to split the two journeys (Finsbury Park to Willesdon Junction via West Hampstead), then the Zones 1-2 travelcard would just be cheaper, but buying the through ticket and breaking the journey would allow me to use
Oyster▸ for the other journeys and would actually be cheaper (^8.80) than the ^12.10 travelcard.